Preheat Oven:

Preheat to 375°F (190°C) and lightly grease a 9x13-inch baking dish.

Brown the Beef:

In a large skillet, heat olive oil over medium heat. Add ground beef and cook until browned, about 6–8 minutes. Drain excess grease if needed.

Add Aromatics & Zucchini:

Add onion and cook 2–3 minutes until softened. Stir in garlic and zucchini, and sauté for 5–6 minutes until the zucchini starts to soften.

Stir in Tomatoes & Seasoning:

Add diced tomatoes with their juices, tomato sauce, Italian seasoning, salt, pepper, and red pepper flakes (if using). Simmer for 5–7 minutes to blend flavors.

Assemble the Casserole:

Spoon the beef and zucchini mixture into the prepared baking dish. Top evenly with mozzarella and sprinkle with Parmesan.

Bake:

Bake uncovered for 25–30 minutes, or until cheese is melted and bubbly. Let rest 5–10 minutes before serving.

Garnish (Optional):

Sprinkle with chopped basil or parsley just before serving.