Preheat your oven to 375°F (190°C).
Heat the olive oil in a large skillet over medium heat. Cook the onion until softened, then add the garlic and cook for another 30 seconds.
Add the ground beef and cook until browned. Drain any excess grease.
Stir in the tomato sauce, Italian seasoning, salt, and pepper. Simmer for 5 minutes.
Arrange half of the zucchini slices in a greased baking dish.
Spoon half of the beef mixture over the zucchini and top with half of the cheddar and mozzarella.
Repeat the layers, then sprinkle Parmesan cheese over the top.
Bake for 30-35 minutes, or until the cheese is golden and bubbling.
Let the casserole rest for 10 minutes before serving.