01 - Preheat oven to 400°F. Line a baking dish with parchment paper or lightly coat with olive oil.
02 - Slice the zucchini in half lengthwise. Using a spoon, gently scoop out centers to create boats, leaving a 1/4-inch shell. Finely chop 1/2 cup of the scooped zucchini flesh and set aside.
03 - Heat olive oil in a large skillet over medium heat. Add diced onion and sauté for 2 minutes until translucent. Add garlic, chopped mushrooms, and reserved zucchini flesh. Cook for 3 to 4 minutes until tender.
04 - Add ground turkey to the skillet. Cook, breaking up the meat with a spatula, until browned and cooked through, about 5 to 6 minutes. Stir in salt, black pepper, and red pepper flakes if desired.
05 - Remove skillet from heat. Mix in basil pesto and half of the Parmesan cheese until evenly combined.
06 - Arrange zucchini boats in the prepared baking dish. Evenly spoon turkey-mushroom mixture into each boat.
07 - Sprinkle mozzarella and the remaining Parmesan cheese over the filling.
08 - Bake for 20 to 25 minutes, or until zucchini is tender and cheese is golden and bubbling.
09 - Remove from oven, garnish with chopped basil, and serve warm.