01 - Preheat oven to 400°F. Line a baking dish with parchment paper or lightly grease it.
02 - Using a spoon, carefully scoop out the centers of each zucchini half to form boats. Reserve about half of the scooped flesh and finely chop it for the filling.
03 - Heat olive oil in a large skillet over medium heat. Add the diced onion and cook until translucent, about 3 minutes. Add minced garlic and cook for 30 seconds until fragrant.
04 - Add ground turkey to the skillet, breaking it apart with a spoon. Cook until browned and cooked through, about 5 to 6 minutes.
05 - Stir in the chopped zucchini flesh and chopped spinach. Cook until the spinach is wilted, about 2 minutes.
06 - Remove the skillet from heat. Mix in the basil pesto, half of the mozzarella, and half of the Parmesan. Season with salt, pepper, and red pepper flakes if desired.
07 - Arrange the zucchini boats in the prepared baking dish. Spoon the turkey and spinach filling evenly into each boat.
08 - Sprinkle the remaining mozzarella and Parmesan evenly over the filled zucchini boats.
09 - Bake for 20 to 25 minutes, or until the zucchini is tender and the cheese is golden and bubbly.
10 - Remove from the oven and garnish with fresh basil leaves before serving.