01 - Preheat the oven to 375°F. Line a loaf pan with parchment paper or lightly grease it to prevent sticking.
02 - In a large mixing bowl, combine the ground chicken, Parmesan cheese, mozzarella cheese, chopped onion, minced garlic, parsley, breadcrumbs, beaten egg, milk, marinara sauce, Italian seasoning, salt, and pepper. Mix gently until just combined, being careful not to overwork the meat.
03 - Transfer the mixture into the prepared loaf pan and gently press it down to form an even, compact loaf shape.
04 - Spread a few additional tablespoons of marinara sauce evenly across the top of the loaf for extra flavor and to help retain moisture during baking.
05 - Bake for 45 to 50 minutes, or until the meatloaf is golden brown on top, feels set when pressed, and the internal temperature registers 165°F on an instant-read thermometer.
06 - Remove from the oven and let the meatloaf rest in the pan for 10 minutes before slicing. This allows the juices to redistribute. Serve with additional warmed marinara sauce on the side if desired.