Preheat and prep the dish: Heat the oven to 400°F (200°C).
Lightly grease a 9x13-inch baking dish with cooking spray or a drizzle of olive oil.
Hollow the zucchini: Trim the stems and slice each zucchini in half lengthwise. Use a spoon to scoop out the centers, leaving about 1/4 inch of flesh to create “boats.” Reserve about half of the scooped zucchini flesh, finely chop it, and set aside.
Par-bake the boats: Arrange the zucchini halves cut side up in the baking dish. Sprinkle with a pinch of salt and brush with a little olive oil.
Bake for 8–10 minutes to soften slightly. This helps them cook evenly later.
Sauté the aromatics: While the boats bake, warm 1 tablespoon olive oil in a large skillet over medium heat. Add diced onion and cook 3–4 minutes until translucent.
Stir in garlic and cook 30 seconds until fragrant.
Cook the turkey: Add ground turkey to the skillet. Season with salt, pepper, Italian seasoning, smoked paprika, and red pepper flakes if using. Break it up with a spoon and cook until browned and no longer pink, about 6–8 minutes.
Add zucchini and sauce: Stir in the chopped zucchini flesh.
Cook 2–3 minutes to release moisture. Pour in the marinara sauce and simmer 2–3 minutes until slightly thickened. Taste and adjust seasoning.
Fill and top: Spoon the turkey mixture into the par-baked zucchini shells, packing it in evenly.
Sprinkle mozzarella and Parmesan over the top.
Bake to finish: Return the dish to the oven and bake 12–15 minutes, until the zucchini is tender and the cheese is melted and lightly golden.
Garnish and serve: Let cool for 5 minutes. Sprinkle with fresh parsley or basil. Serve warm with a simple side salad or garlic bread if you want something extra.