01 - Preheat oven to 400°F.
02 - Slice zucchini in half lengthwise and scoop out flesh, leaving a ¼-inch thick shell. Chop the scooped flesh and set aside.
03 - Arrange zucchini halves in a baking dish, brush with 1 tbsp olive oil, and season lightly with salt and black pepper.
04 - Heat remaining 1 tbsp olive oil in a large skillet over medium heat. Sauté onion for 2–3 minutes until softened, then add garlic and cook 1 minute more.
05 - Add ground beef to skillet, breaking up with a spoon. Cook about 5 minutes until browned, then drain excess fat.
06 - Stir in chopped zucchini flesh, diced tomatoes, tomato paste, oregano, basil, salt, black pepper, and red pepper flakes. Simmer 5–7 minutes until most liquid evaporates.
07 - Spoon beef mixture into zucchini halves, mounding slightly.
08 - Sprinkle each zucchini boat evenly with mozzarella and Parmesan cheeses.
09 - Bake for 20–25 minutes until zucchini is tender and cheese is golden and bubbling.
10 - Allow to cool for 5 minutes before serving.