Preheat the oven to 375°F (190°C).
Heat olive oil in a large skillet over medium heat.
Add the chopped onion and cook for 3-4 minutes until softened.
Add the minced garlic and cook for 1 minute until fragrant.
Add the ground beef to the skillet and cook until browned, breaking it up with a spoon, about 6-8 minutes.
Drain excess fat from the skillet.
Stir in the diced tomatoes, tomato sauce, dried oregano, dried basil, salt, and black pepper. Simmer for 5 minutes, stirring occasionally.
Lightly grease a 9×9 inch baking dish.
Arrange half of the sliced zucchini in an even layer on the bottom of the baking dish.
Spread half of the ground beef mixture evenly over the zucchini layer.
Sprinkle half of the shredded mozzarella cheese over the beef layer.
Add the remaining zucchini slices in an even layer on top.
Spread the remaining ground beef mixture over the zucchini.
Sprinkle the remaining mozzarella cheese and the grated Parmesan cheese evenly over the top.
Cover the baking dish with aluminum foil and bake for 30 minutes.
Remove the foil and bake for an additional 15 minutes until the cheese is golden and bubbly.
Let the casserole rest for 5 minutes before serving.