Preheat oven to 375°F (190°C) and lightly grease a 9×13-inch baking dish.
- In a large skillet over medium-high heat, cook ground beef until browned, breaking it apart with a spoon. Drain excess fat if needed.
- Add onion and garlic to the beef and sauté until softened.
- Stir in diced tomatoes, tomato sauce, Italian seasoning, salt, and pepper. Simmer for 5 minutes.
- In a separate skillet, sauté zucchini slices in olive oil or butter for 3–4 minutes until slightly tender.
- Layer half of the zucchini slices in the baking dish. Spread half of the beef mixture over the zucchini, then sprinkle with half of the cheddar and mozzarella cheese.
- Repeat with remaining zucchini, beef mixture, and cheese.
- Cover with foil and bake for 20 minutes. Remove foil and bake for an additional 10–15 minutes, until cheese is melted and bubbly.
- Let stand for 5 minutes before serving.