Preheat oven to 375°F (190°C). Grease a standard 9x5-inch loaf pan or line it with parchment paper, leaving overhang on the sides for easy removal.

In a large bowl, combine ground chicken, shredded mozzarella, grated Parmesan, minced garlic, breadcrumbs, egg, parsley, Italian seasoning, salt, pepper, and red pepper flakes if using. Mix with your hands until just evenly combined, about 60 to 90 seconds. Do not overmix.

Transfer the mixture to the prepared loaf pan and press gently into an even shape without compacting it tightly. Alternatively, shape into a free-form loaf on a parchment-lined rimmed baking sheet for a crispier exterior.

Bake for 35 to 40 minutes until set through and beginning to brown at the edges.

Sprinkle extra mozzarella and Parmesan evenly over the top of the loaf. Return to the oven and bake for a final 10 minutes until the cheese is fully melted, golden, and beginning to crisp at the edges.

Check the internal temperature with an instant-read thermometer inserted into the center of the loaf. It must reach 165°F before removing from the oven.

Remove from the oven and let the meatloaf rest in the pan for a full 10 minutes before slicing. Use a sharp serrated knife and slice with a gentle sawing motion to keep the cheese topping and loaf intact.