01 - Preheat oven to 400°F. Line a baking dish with parchment paper or lightly grease it.
02 - Slice the zucchini in half lengthwise. Use a spoon to scoop out the centers, leaving a 1/4-inch border to form boats. Chop the scooped-out zucchini flesh and set aside.
03 - Heat olive oil in a large skillet over medium heat. Add onion and cook for 2 to 3 minutes until softened. Add garlic and cook for 30 seconds until fragrant.
04 - Add ground turkey to the skillet. Cook, breaking it up with a spoon, until no longer pink, about 5 minutes.
05 - Stir in mushrooms and chopped zucchini flesh. Cook for 4 to 5 minutes until vegetables are softened and most moisture has evaporated.
06 - Stir in pesto, salt, black pepper, and red pepper flakes if using. Remove from heat and mix in half the chopped basil.
07 - Arrange zucchini boats in the prepared baking dish. Fill each boat generously with the turkey and mushroom mixture.
08 - Sprinkle mozzarella and Parmesan cheese evenly over the filled boats.
09 - Bake for 20 to 25 minutes until zucchini is tender and cheese is melted and golden.
10 - Garnish with remaining fresh basil before serving.