Preheat your oven to 375°F (190°C). Lightly grease a standard 9x5 inch loaf pan.

In a large mixing bowl, pour the 1/2 cup of whole milk over the breadcrumbs. Let this sit for about 5 minutes until the breadcrumbs soak up the milk, creating a porridge-like consistency.

Add the minced onion, beaten egg, minced garlic cloves, chopped fresh parsley, Italian seasoning, salt, and black pepper to the wet breadcrumb mixture. Whisk everything together lightly until well combined.

Add the ground chicken and finely grated Parmesan cheese to the bowl. Use clean hands to gently mix all the ingredients until just combined and uniform. Avoid overmixing to prevent toughness.

Transfer the mixture to your prepared loaf pan, forming it into a compact loaf shape. Press a small, shallow depression into the center of the loaf with your thumb to help cook evenly.

Bake the meatloaf for 40 minutes. If using the optional glaze, brush the marinara sauce over the top. Return to the oven and continue baking for another 15-20 minutes, or until the internal temperature reaches 165°F (74°C).

Remove from the oven and let the meatloaf rest in the pan for 10-15 minutes to redistribute juices. Slice thickly and serve.