Set your oven to preheat at 375°F (190°C). Lightly coat a 9x5-inch loaf tin with cooking spray or line it with parchment paper for easier removal.
In a large mixing bowl, gently fold together the ground chicken, breadcrumbs, grated Parmesan, egg, milk, garlic, chopped onion, parsley, Italian herbs, salt, and pepper. Take care not to overmix to preserve tenderness.
Transfer the mixture into the prepared loaf pan, pressing it in evenly to form a uniform loaf shape.
Place the pan in the oven and bake for 40 to 45 minutes, or until the meatloaf is nearly cooked through but not browned on top.
If using, spoon the marinara sauce evenly across the top and sprinkle with the mozzarella cheese. Return to the oven and continue baking for an additional 10 to 15 minutes, or until the internal temperature reaches 165°F (74°C).
Allow the meatloaf to rest undisturbed for 5 to 10 minutes after baking to retain juices. Slice and serve warm. Garnish with a sprinkle of extra Parmesan or fresh parsley if desired.