Preheat oven to 375°F (190°C).
Slice zucchini lengthwise and scoop out the centers, leaving about a ¼-inch border.
Place zucchini cut-side up on a baking sheet, brush lightly with olive oil, and season with salt and pepper.
Bake for 10–12 minutes, then remove and blot any excess moisture.
In a bowl, combine chicken, marinara sauce, garlic powder, Italian seasoning, red pepper flakes, mozzarella, and Parmesan.
Spoon the chicken mixture evenly into each zucchini half.
Sprinkle remaining mozzarella and Parmesan over the tops.
Return to the oven and bake uncovered for 20–25 minutes, until cheese is melted and lightly golden.
For extra browning, broil for 2–3 minutes at the end.
Let rest for 5 minutes before serving.