Step 1: Preheat your oven to 375°F (190°C) and grease a large baking dish.
Step 2: Slice each zucchini in half lengthwise. Use a spoon to scoop out the centers, leaving a 1/4-inch thick shell. Chop the scooped-out flesh and set aside.
Step 3: Place the zucchini shells in the baking dish and bake for 10-12 minutes to slightly soften them.
Step 4: In a large skillet over medium heat, cook the ground turkey or sausage with the diced onion until the meat is browned.
Step 5: Add the garlic and the chopped zucchini flesh to the skillet. Cook for another 3-5 minutes until the vegetables are tender.
Step 6: Stir in the marinara sauce and dried oregano. Season with salt and pepper. Let the mixture simmer for 2 minutes.
Step 7: Spoon the meat mixture into the pre-baked zucchini shells, mounding it slightly.
Step 8: Top each boat with mozzarella and parmesan cheese. Return to the oven and bake for 15-20 minutes, or until the cheese is melted and bubbly.
Step 9: Garnish with fresh basil before serving.