01 - Preheat the oven to 400°F. Lightly grease a large baking dish with cooking spray or olive oil.
02 - Slice each zucchini in half lengthwise. Using a spoon, carefully scoop out the center flesh, leaving about a 1/4-inch shell to maintain structural integrity. Reserve the scooped flesh and roughly chop it; set aside.
03 - Heat 1 tablespoon of olive oil in a large skillet over medium heat. Add the diced onion and cook for 2 to 3 minutes until softened and translucent. Add the minced garlic and cook for 30 seconds, stirring constantly to prevent burning.
04 - Add the ground turkey to the skillet and cook, breaking it apart with a wooden spoon, until no longer pink throughout, approximately 5 minutes.
05 - Stir in the chopped zucchini flesh and baby spinach. Continue cooking until the spinach has completely wilted and any excess moisture has evaporated, about 2 minutes.
06 - Remove the skillet from heat. Fold in the basil pesto, half of the mozzarella, and half of the Parmesan. Season with salt, black pepper, and red pepper flakes if desired. Stir until evenly combined.
07 - Arrange the hollowed zucchini shells in the prepared baking dish. Drizzle with the remaining 1 tablespoon of olive oil and sprinkle lightly with a pinch of salt. Fill each boat generously with the turkey and spinach mixture, mounding slightly. Top with the remaining mozzarella and Parmesan.
08 - Bake for 20 to 22 minutes, or until the zucchini shells are fork-tender and the cheese is melted, golden, and bubbly.
09 - Remove from the oven and garnish with fresh sliced basil. Serve immediately while hot.