Heat the oven to 400 °F. Cut each zucchini in half lengthwise and scoop out the center with a spoon, leaving a 1/4-inch shell. Chop the scooped flesh (about 1 cup) and set aside. Arrange the shells cut-side up on a rimmed baking sheet.
Heat the olive oil in a large skillet over medium-high. Add the ground turkey, onion, 1/2 teaspoon of the salt and the pepper. Cook 7-8 minutes, breaking up the meat, until no longer pink.
Add the garlic and Italian seasoning and cook 30 seconds. Stir in the chopped zucchini flesh and cook 3 minutes, until softened. Pour in 1 cup of the marinara and simmer 2 minutes.
Spoon the turkey mixture into the zucchini shells, mounding it slightly. Top with the remaining 1/2 cup marinara and the mozzarella.
Bake 20-22 minutes, until the zucchini is tender and the cheese is bubbling and lightly browned.
Rest 5 minutes, then finish with the Parmesan.
