1. Preheat oven to 375°F (190°C). Slice zucchini into thin rounds and set aside. 2. In a skillet over medium heat, add olive oil and sauté onions until soft. Add garlic, then ground beef. Cook until browned, breaking it up as it cooks. Drain excess fat if needed. Season with salt, pepper, Italian seasoning, and red pepper flakes. Stir in tomato sauce and simmer for 2–3 minutes. 3. In a greased 9×13 baking dish, layer half of the zucchini rounds. Spread half of the beef mixture over the zucchini. Sprinkle with half the mozzarella and Parmesan. Repeat layers with remaining zucchini, beef, and cheeses. 4. Cover with foil and bake for 20 minutes. Remove foil and bake another 10–15 minutes, until cheese is bubbly and golden. 5. Let stand 5 minutes before slicing and serving. Garnish with fresh herbs if desired.