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, breaking it up with a spoon, until browned and cooked through, about 6-8 minutes.
Drain any excess fat from the skillet.
Stir in the diced zucchini, diced tomatoes with their juice, tomato sauce, dried oregano, dried basil, salt, and black pepper.
Simmer the mixture for 10 minutes, stirring occasionally.
Transfer the beef and zucchini mixture to a 9×13-inch baking dish and spread evenly.
Sprinkle the shredded mozzarella cheese evenly over the top.
Sprinkle the grated Parmesan cheese over the mozzarella.
Bake in the preheated oven for 25-30 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let the casserole rest for 5 minutes before serving.