This high-protein ground beef casserole is a low-carb dinner that keeps you full and satisfied. Made with ground beef, cheese, and veggies, it's perfect for busy weeknights. Save this easy…

Ingredients

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Instructions

Step 1

Preheat your oven to 375°F (190°C). This moderate temperature allows the casserole to cook through evenly without drying out the ground beef or burning the cheese topping. While the oven heats, gather all your ingredients and prep your vegetables: dice the onion, mince the garlic, dice the zucchini and bell pepper into uniform 1/2-inch pieces (uniformity ensures even cooking), and slice the mushrooms. Draining the diced tomatoes is important to prevent the casserole from becoming watery; set them aside. Shred the mozzarella if not pre-shredded (freshly shredded melts better). This mise en place approach streamlines the cooking process.

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Step 2

Heat 1 tablespoon of olive oil in a large oven-safe skillet (12-inch cast iron or stainless steel) over medium-high heat. Once the oil shimmers, add the ground beef. Break it apart with a wooden spoon and cook for 5-7 minutes, stirring occasionally, until browned. The beef should be crumbly with no pink remaining. Do not overcrowd the pan; if necessary, cook in batches to ensure proper browning rather than steaming. Once browned, use a slotted spoon to transfer the beef to a plate, leaving about 1 tablespoon of rendered fat in the skillet (if using lean beef, add a bit more oil). This fond (browned bits) adds deep flavor to the vegetables.

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Step 3

In the same skillet, add the diced onion and cook over medium heat for 3-4 minutes until translucent, scraping up any browned bits from the bottom. Add the minced garlic and cook for 30 seconds until fragrant. Then add the diced zucchini, bell pepper, and sliced mushrooms. Cook for 5-6 minutes, stirring occasionally, until the vegetables have softened and released some moisture. The zucchini should be tender but not mushy, and the mushrooms should be golden brown. Season with a pinch of salt and pepper to draw out flavors. The moisture from the vegetables will help deglaze the pan, incorporating all the savory browned bits into the mixture.

Step 4

Return the browned ground beef to the skillet with the vegetables. Add the drained diced tomatoes, dried oregano, dried basil, and red pepper flakes if using. Stir everything together until well combined. Pour in the beef broth and bring the mixture to a simmer. Let it cook for 2-3 minutes, allowing the flavors to meld and the liquid to reduce slightly. The mixture should be saucy but not soupy. Taste and adjust seasoning with salt and pepper as needed. Remove the skillet from heat. If your skillet is not oven-safe, transfer the mixture to a greased 9×13-inch baking dish.

Step 5

Sprinkle the shredded mozzarella cheese evenly over the top of the beef and vegetable mixture, followed by the grated Parmesan cheese. The Parmesan adds a nutty, salty crust, while the mozzarella provides that irresistible gooey stretch. Place the skillet or baking dish in the preheated oven and bake for 20 minutes, until the cheese is melted, bubbly, and golden brown in spots. For a deeper golden crust, you can switch to broil for the last 1-2 minutes, but watch carefully to prevent burning. Let the casserole rest for 5 minutes before serving to allow the juices to settle and the cheese to set slightly.

For best results

  • Choosing the right ground beef: For the best balance of flavor and fat content, use ground beef with 85/15 or 90/10 lean-to-fat ratio. Higher fat (like 70/30) will make the casserole greasy, while extra-lean (93/7 or 96/4) can result in a dry texture. If you opt for leaner beef, consider adding a tablespoon of olive oil when browning to compensate for moisture. Always drain excess fat after browning if using higher-fat beef, but leave a little for flavor. Grass-fed beef offers a richer, more complex taste and higher omega-3 content, but any quality ground beef works.
  • Vegetable prep and moisture control: Low-carb vegetables like zucchini and mushrooms release a lot of water during cooking. To avoid a watery casserole, make sure to cook them thoroughly in the skillet until most of the moisture has evaporated before adding the broth. You can also salt the diced zucchini and let it sit for 10 minutes, then pat dry with paper towels to draw out excess liquid. For added texture, you can roast the vegetables separately at 400°F for 15 minutes before adding to the casserole, which intensifies their flavor and reduces moisture.
  • Cheese melting secrets: For the best melt, always shred cheese from a block rather than using pre-shredded bags, which contain anti-caking agents (like cellulose) that prevent smooth melting. Mozzarella gives that classic stretch, while Parmesan adds a savory crust. Feel free to mix in other cheeses like provolone, fontina, or a sharp cheddar for different flavor profiles. For a dairy-free version, use a high-quality vegan mozzarella shred that melts well, and nutritional yeast in place of Parmesan.
  • Make it ahead and meal prep: This casserole is perfect for meal prep. Assemble the casserole completely (without baking) up to 2 days in advance, cover tightly with foil, and refrigerate. When ready to bake, add 10-15 minutes to the baking time since starting from cold. You can also freeze the unbaked casserole for up to 3 months; thaw overnight in the refrigerator before baking. Baked leftovers keep in the fridge for up to 5 days and reheat beautifully in the microwave or oven (covered at 350°F for 15 minutes).
  • Boost the protein even more: To increase the protein content without adding many carbs, stir in 1 cup of cottage cheese or ricotta into the beef mixture before topping with mozzarella. Alternatively, mix in a scoop of unflavored collagen peptides or egg white protein powder (dissolved in a bit of broth) for an extra 10-15g protein per serving. For a heartier casserole, add a layer of cooked cauliflower rice or riced broccoli between the beef mixture and cheese for volume and fiber without the carbs.
  • Adjusting for spice and flavor: This casserole is mild but can be customized to your heat preference. Add 1/2 teaspoon of smoked paprika or chipotle powder for a smoky kick, or stir in a tablespoon of tomato paste for deeper umami. Fresh herbs like basil or oregano can be used instead of dried; use triple the amount (e.g., 1 tbsp fresh oregano = 1 tsp dried). For a Italian twist, add 1/2 cup of sliced black olives and a sprinkle of Italian seasoning. For a Tex-Mex version, replace oregano and basil with cumin and chili powder, and top with pepper jack cheese.

Variations

  • Keto and low-carb adaptation: This recipe is already low-carb, but to make it strictly ketogenic (under 5g net carbs per serving), swap the bell pepper for additional zucchini or spinach, and use full-fat mozzarella and Parmesan. Replace the diced tomatoes with 1/2 cup of sugar-free tomato sauce or a few tablespoons of tomato paste mixed with water. The beef broth should be unsalted and low-carb (check labels). Serve with a side of steamed broccoli or a green salad with avocado to keep carbs minimal while adding healthy fats.
  • Vegetarian high-protein version: For a meatless option, substitute the ground beef with 1 pound of plant-based ground meat (like Beyond Meat or Impossible) or crumbled extra-firm tofu (press and crumble, then sauté until golden). Alternatively, use 2 cups of cooked lentils or a mixture of black beans and quinoa for plant-based protein. The vegetable base remains the same. To boost protein, add 1 cup of cottage cheese or a dollop of Greek yogurt on top before serving. Nutritional yeast can replace Parmesan for a cheesy, nutty flavor without dairy.
  • Turkey or chicken casserole: Lean ground turkey (93/7) or ground chicken works wonderfully in this recipe. Since poultry is leaner, add an extra tablespoon of olive oil when browning to prevent dryness. You can also use ground pork or a mix of beef and pork for a richer flavor. For a completely different twist, use shredded rotisserie chicken (about 3 cups) and skip the browning step; simply sauté the vegetables, then stir in the chicken and proceed. Adjust seasoning accordingly—turkey pairs well with sage and thyme, while chicken loves rosemary and lemon zest.
  • Spicy Southwestern variation: Transform this casserole into a Southwestern fiesta by replacing the oregano and basil with 1 teaspoon of ground cumin, 1 teaspoon of chili powder, and 1/2 teaspoon of smoked paprika. Add 1 can (4 oz) of diced green chiles and 1/2 cup of corn (if not strict low-carb) or diced jicama for crunch. Top with a blend of Monterey Jack and cheddar cheese instead of mozzarella. Serve with sliced avocado, fresh cilantro, and a dollop of sour cream or Greek yogurt.
  • Italian-inspired variation: For an Italian flair, use ground Italian sausage (sweet or hot) in place of or alongside the ground beef. Add 1/2 cup of sliced black olives, 1 tablespoon of tomato paste, and 1/2 teaspoon of fennel seeds. Use a blend of provolone and mozzarella cheese, and sprinkle with fresh basil after baking. Serve over zucchini noodles or cauliflower gnocchi for a complete low-carb Italian dinner.

Nutrition

  • Calories385 kcal
  • Total Fat22g
  • Saturated Fat10g
  • Cholesterol85mg
  • Sodium620mg
  • Total Carbohydrates12g
  • Dietary Fiber3g
  • Sugars6g
  • Protein35g

Common Questions

Can I make this casserole dairy-free?
Yes, you can easily make this casserole dairy-free. Use a high-quality vegan mozzarella shred that melts well, such as brands made with coconut oil or potato starch. For the Parmesan flavor, substitute nutritional yeast (use about 3 tablespoons) which provides a cheesy, nutty taste without dairy. Ensure your beef broth is dairy-free (most are, but check labels). The rest of the ingredients are naturally dairy-free. The texture will be slightly different but still delicious and satisfying.
How do I store leftovers and how long do they last?
Store leftovers in an airtight container in the refrigerator for up to 5 days. To reheat, microwave individual portions for 1-2 minutes or reheat in a 350°F oven covered with foil for about 15 minutes. The casserole also freezes well: portion into freezer-safe containers or wrap tightly in foil and freeze for up to 3 months. Thaw overnight in the refrigerator before reheating. For best texture, avoid freezing after adding cheese topping; instead, freeze the base and add fresh cheese when baking.
Can I add rice or pasta to make it more filling?
If you’re not strictly low-carb, you can add cooked rice, quinoa, or pasta to make the casserole heartier. Stir in 1-2 cups of cooked cauliflower rice for a low-carb option, or use regular rice (brown or white) and increase the broth slightly to keep it moist. For pasta, use small shapes like elbows or shells; cook al dente and fold into the mixture before baking. Note that adding grains will increase the carbohydrate content, so adjust your macros accordingly.
What can I use instead of ground beef?
Ground beef can be swapped with ground turkey, chicken, pork, lamb, or bison. For a vegetarian option, use plant-based ground meat, crumbled tofu, cooked lentils, or a mix of black beans and quinoa. If using a leaner protein like turkey, add a tablespoon of olive oil to prevent dryness. For extra flavor, you can also use half ground beef and half ground pork or Italian sausage. Adjust seasonings to complement the chosen protein.
Is this casserole gluten-free?
Yes, this casserole is naturally gluten-free as written. All ingredients are gluten-free: ground beef, fresh vegetables, cheese, and broth. However, always check labels on canned tomatoes and broth to ensure no gluten-containing additives. If you are highly sensitive, use certified gluten-free broth and diced tomatoes. The recipe contains no wheat, flour, or breadcrumbs, so it’s safe for those with celiac disease or gluten intolerance.
Can I prepare this casserole in advance for meal prep?
Absolutely! This casserole is perfect for meal prep. You can fully assemble it (without baking) up to 2 days ahead and store it covered in the refrigerator. When ready to bake, add 10-15 minutes to the baking time since starting from cold. Alternatively, bake it completely, let it cool, then portion into meal prep containers. It reheats beautifully and tastes even better the next day as flavors meld. For freezing, assemble without baking, wrap tightly, and freeze for up to 3 months.
What can I serve with this casserole to keep it low-carb?
This casserole is a complete meal on its own with protein, vegetables, and healthy fats. However, if you’d like a side, try a simple green salad with a vinaigrette dressing, steamed broccoli or asparagus, roasted Brussels sprouts, or cauliflower rice. For extra healthy fats, add sliced avocado or a dollop of sour cream. Avoid starchy sides like bread, potatoes, or rice to keep the meal low-carb and high-protein.
How can I reduce the sodium content?
To lower sodium, use low-sodium or no-salt-added beef broth and diced tomatoes. Choose cheeses labeled low-sodium, or reduce the amount of Parmesan (which is naturally high in salt). Season with herbs and spices instead of salt, and avoid adding extra salt until the end, then taste and adjust. You can also rinse canned vegetables if using. The natural flavors of the beef and vegetables will still shine through.