Preheat your oven to 375°F (190°C).

In a large skillet, heat olive oil over medium heat. Add chopped onions and minced garlic, and sauté for 3-4 minutes until translucent.

Introduce the ground beef to the skillet and cook for 5-7 minutes until browned, stirring occasionally.

Stir in the marinara sauce, oregano, salt, pepper, and red pepper flakes, and let simmer for 5 minutes.

In a bowl, mix ricotta cheese with half of the mozzarella until smooth.

Grease a baking dish and layer with zucchini slices, half of the beef mixture, the cheese mixture, and repeat layers finishing with zucchini and beef.

Top with remaining mozzarella and a layer of Parmesan cheese.

Bake for 25-30 minutes until cheese is melted and bubbly.

Let cool for 5 minutes before serving.