This one-pan protein casserole makes easy weeknight meals a breeze with minimal cleanup. Packed with ground beef, cheese, and veggies, it's a macro-friendly meal the whole family will love.

Ingredients

• 1 lb (450g) lean ground beef (85/15 or 90/10)
• 1 medium onion, diced (about 1 cup)
• 2 cloves garlic, minced
• 1 medium zucchini, diced (about 1.5 cups)
• 1 red bell pepper, diced (about 1 cup)
• 1 cup (150g) cherry tomatoes, halved
• 1 cup (240ml) low-sodium beef broth
• 1 can (14.5 oz/411g) diced tomatoes, drained
• 1 tsp dried oregano
• 1 tsp smoked paprika
• 1/2 tsp salt
• 1/4 tsp black pepper
• 1.5 cups (170g) shredded reduced-fat cheddar cheese
• 2 tbsp chopped fresh parsley (for garnish)

Instructions

Step 1

Preheat your oven to 375°F (190°C). While the oven is heating, heat a large oven-safe skillet (or a cast-iron pan) over medium-high heat. Add the ground beef and cook, breaking it apart with a wooden spoon, until browned and no longer pink, about 5-7 minutes. The beef should sizzle as it hits the pan; if it doesn’t, the pan isn’t hot enough. Browning creates deep savory flavor through the Maillard reaction. Once cooked, use a slotted spoon to transfer the beef to a plate, leaving about 1 tablespoon of rendered fat in the pan (if using lean beef, add a drizzle of olive oil).

Step 2

In the same skillet over medium heat, add the diced onion and cook for 2-3 minutes until softened and translucent, scraping up any browned bits from the bottom of the pan (this is where flavor lives!). Add the minced garlic and cook for 30 seconds until fragrant, stirring constantly to prevent burning. The aroma should be sweet and savory. Next, add the diced zucchini and red bell pepper. Sauté for 4-5 minutes, stirring occasionally, until the vegetables are tender but still have a bit of crunch. The zucchini will release some moisture, which helps create a saucy base.

Step 3

Return the cooked ground beef to the skillet. Add the halved cherry tomatoes, drained diced tomatoes, beef broth, dried oregano, smoked paprika, salt, and black pepper. Stir everything together until well combined. Bring the mixture to a gentle simmer, then reduce the heat to low and let it cook for 5 minutes, allowing the flavors to meld. The liquid should reduce slightly, thickening into a rich, savory sauce. Taste and adjust seasoning if needed. If the mixture seems too dry, add a splash more broth.

Step 4

Remove the skillet from the heat. Sprinkle the shredded cheddar cheese evenly over the top, covering the entire casserole surface. For best results, use reduced-fat cheese that melts well—full-fat cheese also works but adds more calories. Transfer the skillet to the preheated oven and bake for 15-20 minutes, until the cheese is melted, bubbly, and lightly golden around the edges. The casserole should be hot throughout, with the cheese forming a delicious crust. If you prefer a browner top, switch to broil for the last 2 minutes, watching carefully to avoid burning.

Step 5

Remove the skillet from the oven and let it rest for 5 minutes before serving. This allows the casserole to set, making it easier to portion. Garnish with fresh chopped parsley for a pop of color and freshness. Serve directly from the skillet, scooping generous portions onto plates. This one-pan meal pairs beautifully with a side salad or steamed green beans for extra veggies. Enjoy the combination of savory beef, tender vegetables, and gooey melted cheese in every bite.

For best results

• Choose the right ground beef: For a macro-friendly dish, opt for lean ground beef with 85/15 or 90/10 lean-to-fat ratio. This keeps the calorie and saturated fat content in check while still providing plenty of protein and flavor. If you use extra-lean beef (93/7 or higher), add a teaspoon of olive oil when browning to prevent dryness and ensure good browning. The fat content also affects how the casserole holds together; too lean and it may be crumbly, so balance is key.
• Maximize vegetable variety: Feel free to swap or add other low-carb vegetables like mushrooms, spinach, or cauliflower. Mushrooms add an earthy umami flavor, while spinach wilts down nicely and boosts iron content. If using frozen vegetables, thaw and pat them dry first to avoid excess moisture that can make the casserole watery. Dicing vegetables into uniform pieces ensures even cooking and a pleasant texture in every bite.
• Cheese melting secrets: For a perfectly melted and bubbly cheese topping, shred your own cheese from a block rather than using pre-shredded cheese. Pre-shredded varieties often contain anti-caking agents (like cellulose) that inhibit smooth melting. Also, let the cheese come to room temperature for about 10 minutes before sprinkling it on the casserole; this helps it melt more evenly and quickly in the oven. If you want a crispy cheese crust, increase the oven temperature to 400°F for the last 5 minutes of baking.
• Make it ahead for meal prep: This casserole is excellent for meal prep. Assemble the entire dish (without the cheese topping) up to 2 days in advance and refrigerate. When ready to bake, add the cheese and extend the baking time by 5-10 minutes to account for the cold start. Alternatively, bake the casserole completely, let it cool, and portion it into airtight containers. It will keep in the refrigerator for up to 4 days, and the flavors actually deepen overnight. Reheat individual portions in the microwave or oven until hot.
• Boost protein without extra meat: To increase the protein content without adding more meat, stir in a scoop of unflavored collagen peptides or whey protein powder into the broth before adding it to the skillet. This seamlessly blends in and adds about 10 grams of protein per serving. Another option is to mix in 1/2 cup of cottage cheese (blended until smooth) along with the diced tomatoes; it melts into the sauce, adding creaminess and protein without altering the flavor significantly.
• Prevent a watery casserole: Vegetables like zucchini and tomatoes release moisture during cooking, which can make the casserole watery. To avoid this, salt the diced zucchini and let it sit in a colander for 10 minutes, then pat dry with paper towels before cooking. Also, use canned diced tomatoes that are well-drained; you can even give them a gentle squeeze to remove extra liquid. If the sauce still seems thin after simmering, mix 1 teaspoon of cornstarch with 2 tablespoons of cold water and stir it in, cooking for another minute until thickened.
• Serving suggestions for a complete meal: While this casserole is a complete one-pan meal, you can round it out with a simple side. A crisp green salad with a lemon vinaigrette adds freshness and acidity that cuts through the richness of the cheese. For extra fiber, serve with roasted broccoli or asparagus. If you're not strictly low-carb, a side of quinoa or cauliflower rice makes a great base to soak up the savory juices. For a fun twist, spoon the casserole into lettuce wraps for a crunchy, low-carb taco-style meal.

Variations

• Make it a chicken casserole: Swap the ground beef for 1.5 pounds of boneless skinless chicken thighs or breasts, cut into bite-sized pieces. Brown the chicken in the skillet until golden on all sides (it doesn't need to be cooked through). Proceed with the recipe, but use chicken broth instead of beef broth and add 1/2 teaspoon of dried thyme along with the oregano. The cooking time remains the same. This variation is leaner and pairs well with a sprinkle of Parmesan cheese on top.
• Add a spicy kick: For those who love heat, add 1 diced jalapeño (seeds removed for less heat) along with the onions, and include 1/2 teaspoon of cayenne pepper or red pepper flakes with the spices. You can also use pepper jack cheese instead of cheddar for an extra layer of spiciness. Serve with a dollop of Greek yogurt or sour cream to balance the heat. This version is perfect for spice lovers and adds a metabolism-boosting kick.
• Make it vegetarian: Omit the ground beef and double the vegetables—use mushrooms, zucchini, bell peppers, and spinach. For protein, add 1 can (15 oz) of drained and rinsed black beans or chickpeas, and 1 cup of crumbled firm tofu or tempeh. Use vegetable broth instead of beef broth. The cheese remains the same, or you can use a vegan cheese alternative for a dairy-free option. This vegetarian version still packs plenty of protein and fiber.
• Low-carb/keto adaptation: To make this casserole keto-friendly, use full-fat cheese and ground beef with a higher fat content (80/20). Replace the cherry tomatoes with 1/2 cup of tomato sauce (no sugar added) to reduce carbs. Add 1/2 cup of heavy cream to the broth for extra richness and fat. You can also stir in 1/4 cup of cream cheese for a creamy texture. Serve with cauliflower rice or zucchini noodles to keep carbs minimal. This version is satisfying and fits a ketogenic diet.
• Italian-inspired twist: Transform the flavor profile by using Italian seasoning instead of oregano and smoked paprika, and add 1/4 cup of grated Parmesan cheese to the cheese topping. Use ground turkey or Italian sausage (casings removed) for a different protein. Stir in a handful of chopped fresh basil before serving. This variation tastes like a deconstructed lasagna without the noodles, perfect for Italian food lovers on a low-carb diet.
• Breakfast casserole: Repurpose this recipe for a hearty breakfast by using breakfast sausage instead of ground beef, and adding 4 beaten eggs poured over the vegetable mixture before baking. Omit the broth and diced tomatoes, and use a combination of cheddar and mozzarella. Bake until the eggs are set, about 25-30 minutes. This makes a protein-packed breakfast or brunch dish that can be made ahead and reheated.

Nutrition

• Calories385 kcal
• Total Fat18g
• Saturated Fat8g
• Cholesterol85mg
• Sodium620mg
• Total Carbohydrates12g
• Dietary Fiber3g
• Sugars6g
• Protein35g

Common Questions