01 - Preheat oven to 400°F.
02 - Halve zucchinis lengthwise and scoop out centers, leaving a 1/2 inch shell. Chop reserved flesh.
03 - Arrange zucchini halves in baking dish, drizzle with 1 tbsp olive oil, and season lightly with salt and pepper.
04 - Heat remaining 1 tbsp olive oil in skillet over medium heat. Cook onion until softened, about 2–3 minutes. Add garlic and bell pepper; sauté 2 minutes. Stir in chopped zucchini flesh; cook 2 more minutes.
05 - Add ground beef to skillet, breaking it up with a spoon. Cook until browned, about 5 minutes. Drain excess fat if necessary.
06 - Stir in diced tomatoes, tomato paste, oregano, basil, chili flakes (if using), salt, and pepper. Simmer mixture for 5 minutes until thickened.
07 - Fill zucchini shells with beef mixture, mounding slightly. Evenly top with mozzarella and Parmesan cheeses.
08 - Bake uncovered for 20–25 minutes until zucchini is tender and cheese is golden and bubbling.
09 - Sprinkle with fresh parsley or basil before serving.