01 - Preheat the oven to 400°F. Line a baking sheet or large baking dish with parchment paper.
02 - Slice each zucchini in half lengthwise. Using a spoon, scoop out the flesh, leaving a 1/4-inch border to form boats. Finely chop the scooped flesh and set aside.
03 - Heat olive oil in a large skillet over medium heat. Add the diced onion and cook for 2–3 minutes until softened. Stir in the minced garlic and cook for 30 seconds until fragrant.
04 - Add the ground turkey to the skillet. Cook, breaking it apart with a spoon, until browned and cooked through, about 5–6 minutes.
05 - Stir in the chopped zucchini flesh and spinach. Continue cooking until the spinach has wilted, about 2 minutes.
06 - Remove the skillet from heat. Add the basil pesto, salt, black pepper, and crushed red pepper flakes if using. Mix thoroughly. Fold in half of the mozzarella and half of the Parmesan.
07 - Arrange the zucchini boats on the prepared baking sheet. Generously fill each boat with the turkey and spinach mixture.
08 - Sprinkle the remaining mozzarella and Parmesan evenly over the top of each stuffed boat.
09 - Bake for 20–25 minutes until the zucchini is tender and the cheese is golden and bubbling.
10 - Remove from the oven and garnish with fresh basil and parsley before serving.