Heat oven to 400°F (200°C). Halve zucchini lengthwise. Scoop out a shallow channel with a spoon, leaving a 1/4 inch shell and intact ends. Finely chop about 1 cup of the scooped flesh; reserve excess for another use.

Arrange 8 halves cut side up in a baking dish. Season with 1/4 teaspoon salt and add 2 tablespoons water to the dish, around rather than into the shells. Cover with foil and bake 15 minutes.

Meanwhile heat oil in a skillet over medium heat. Cook onion for 4 minutes. Add turkey and break into small pieces for 6–8 minutes. Add chopped zucchini flesh and cook another 4–5 minutes to evaporate released moisture. Stir in garlic, herbs, remaining salt and pepper for 30 seconds.

Add marinara and simmer uncovered for 5 minutes, until spoonable rather than watery. Verify ground turkey reaches 165°F (74°C). Carefully pour away excess water from the baking dish. Divide filling evenly among the shells.

Sprinkle mozzarella and Parmesan over the filling. Bake uncovered for 15–20 minutes, until zucchini is tender when pierced and the filling is hot throughout, reaching 165°F (74°C). If zucchini is still firm, cover loosely and continue baking.

Rest 5 minutes, add basil and serve two halves per portion. Use a wide spatula to lift each boat without breaking the softened shell.