01 - Set oven temperature to 400°F (200°C).
02 - Halve zucchinis lengthwise and scoop out flesh, leaving a 1/2 inch shell; finely chop the reserved flesh.
03 - Place zucchini halves in a baking dish, drizzle with 1 tbsp olive oil, and season with salt and pepper.
04 - Heat remaining 1 tbsp olive oil in a large skillet over medium heat. Sauté onion for 2 to 3 minutes until softened.
05 - Add minced garlic and diced red bell pepper; cook for 2 minutes. Stir in chopped zucchini flesh and cook an additional 2 minutes.
06 - Add ground beef, breaking it apart with a spoon. Cook 5 minutes until browned, then drain excess fat if necessary.
07 - Mix in diced tomatoes, tomato paste, dried oregano, dried basil, chili flakes if using, and salt and pepper. Simmer 5 minutes until sauce thickens.
08 - Spoon beef mixture into zucchini shells, mounding slightly, then top evenly with shredded mozzarella and grated Parmesan.
09 - Bake uncovered for 20 to 25 minutes until zucchini is tender and cheese is melted and golden.
10 - Sprinkle with fresh parsley or basil before serving.