01 - Preheat oven to 400°F and line a large baking dish with parchment paper or lightly grease.
02 - Slice zucchini in half lengthwise. Using a spoon, carefully scoop out centers to form a 1/4-inch shell. Reserve and finely chop the scooped flesh.
03 - Heat olive oil in a large skillet over medium. Add onion and cook until softened, about 3 minutes. Stir in garlic and cook for 30 seconds.
04 - Add ground turkey to skillet. Cook, breaking up with a spoon, until no longer pink, about 5 minutes.
05 - Stir in reserved zucchini flesh, chopped spinach, Italian herbs, and red pepper flakes. Sauté until spinach is wilted and moisture is evaporated, approximately 3 minutes.
06 - Remove from heat. Blend in pesto, half of the mozzarella, and half of the Parmesan. Season with salt and black pepper.
07 - Arrange zucchini halves in the prepared dish. Spoon a portion of marinara sauce into each, then distribute the turkey-spinach filling evenly.
08 - Sprinkle remaining mozzarella and Parmesan over the filled zucchini.
09 - Bake for 20 to 25 minutes until zucchini is tender and cheese is golden and bubbly.
10 - Let rest for 5 minutes before plating and serving.