Preheat the oven to 375°F (190°C).
Heat 1 tablespoon of olive oil in a large skillet over medium heat.
Add the chopped onion and cook for 3-4 minutes until translucent.
Add the minced garlic and cook for 1 minute until fragrant.
Add the ground beef to the skillet, breaking it apart with a spoon. Cook until browned and no longer pink, about 7-8 minutes.
Stir in the tomato sauce, dried oregano, dried basil, salt, and black pepper. Simmer for 5 minutes, then remove from heat.
Lightly grease a 9×13-inch baking dish with the remaining 1 tablespoon of olive oil.
Arrange a layer of zucchini slices on the bottom of the dish.
Spread half of the ground beef mixture evenly over the zucchini layer.
Add a layer of tomato slices over the beef mixture.
Sprinkle with one-third of the mozzarella cheese.
Repeat layering with the remaining zucchini slices, the rest of the ground beef mixture, and the remaining tomato slices.
Top the casserole with the remaining mozzarella cheese and sprinkle the grated Parmesan cheese evenly on top.
Cover the baking dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden brown.
Let the casserole rest for 5 minutes before serving.