Preheat the oven to 375°F and lightly grease a 9x9 inch baking dish.
Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook for 3 minutes until it softens.
Add the ground beef to the skillet and cook until browned, breaking it up with a spoon as it cooks. Drain any excess fat.
Stir in the minced garlic, oregano, salt, and pepper. Cook for 1 minute until the garlic is fragrant.
Pour the tomato sauce into the skillet and stir everything together. Let it simmer for 5 minutes.
Spread half the sliced zucchini in an even layer in the baking dish. Spoon half the beef mixture over the zucchini.
Repeat with the remaining zucchini and beef mixture. Sprinkle the grated Parmesan cheese evenly over the top.
Bake for 25 minutes until the cheese is golden and the zucchini is tender when pierced with a fork. Let it rest for 5 minutes before serving.