01 - Set oven temperature to 400°F.
02 - Cut zucchinis lengthwise and hollow out centers, keeping 1/2-inch thick shells; finely chop the scooped flesh.
03 - Place zucchini halves in a baking dish, drizzle with 1 tablespoon olive oil, and season with salt and pepper.
04 - Heat remaining 1 tablespoon olive oil in a large skillet over medium heat; soften onion for 2–3 minutes.
05 - Incorporate garlic and bell pepper, sauté for 2 minutes, then add chopped zucchini flesh and cook 2 more minutes.
06 - Add ground beef, break it up, and cook until browned, about 5 minutes; drain excess fat if necessary.
07 - Stir in diced tomatoes, tomato paste, oregano, basil, chili flakes if desired, salt, and pepper; simmer for 5 minutes until thickened.
08 - Spoon beef mixture into zucchini shells, mounding slightly, then evenly top with mozzarella and Parmesan cheeses.
09 - Bake uncovered for 20–25 minutes until zucchini is tender and cheese is golden and bubbly.
10 - Sprinkle with fresh parsley or basil before serving.