In a skillet over medium heat, cook ground meat and onion until the ground beef is browned.

Stir the garlic into the cooked beef. Drain excess fat and return the beef to the skillet. Add half of the marinara sauce as well as the seasonings. Stir well to combine and allow to simmer for about 5 minutes. Remove from heat.

Combine zucchini to the ground beef mixture and stir until combined.

Spread half of the zucchini and meat sauce in the prepared casserole dish. Sprinkle it with half of the cheese.

Repeat the layers one time. Spread the remaining marinara over the top. Sprinkle the remaining mozzarella cheese evenly over the top of the casserole.

Cover the dish with foil and bake in the preheated 375°F (190°C) oven for 30 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is melted and bubbly.