Preheat your oven to 375°F (190°C).

In a large skillet, heat the olive oil over medium heat. Add the ground beef or Italian sausage, breaking it apart with a spatula. Cook until browned and cooked through, about 5-7 minutes. Drain any excess fat.

Stir in the marinara sauce, garlic powder, Italian seasoning, red pepper flakes (if using), salt, and pepper. Let it simmer for 2-3 minutes.

In a 9x13-inch baking dish, layer half of the zucchini slices on the bottom. Spread half of the meat sauce mixture over the zucchini. Sprinkle half of the mozzarella cheese on top.

Repeat the layers with the remaining zucchini, meat sauce, and mozzarella cheese. Finish by sprinkling the grated Parmesan cheese on top.

Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.

Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden brown.

Let the casserole cool for 5-10 minutes before serving. Garnish with fresh basil leaves if desired.