Prepare a parchment-lined baking sheet or lightly greased loaf pan, and heat the oven to 375°F (190°C). The prepared surface helps release the loaf without tearing its tender bottom.

Place the ground chicken, breadcrumbs, Parmesan, mozzarella, garlic, egg, milk, parsley, Italian seasoning, salt, and pepper in a large bowl, then gently fold everything together. Stop when the ingredients look evenly distributed and the mixture feels moist and tacky, before repeated mixing makes it dense.

Form an evenly shaped loaf on the baking sheet or settle the mixture gently into the loaf pan, then bake for 35 to 40 minutes. The loaf should become firm with some browning around the edges, but check its thickest center with a thermometer rather than judging doneness by color.

Scatter the extra shredded mozzarella over the baked loaf and return it to the oven for 5 to 7 minutes. The topping should melt into a bubbly layer while the center reaches the minimum poultry cooking temperature explained below.

Leave the loaf undisturbed for 5 minutes, then cut slices and scatter fresh parsley over them. The rested loaf should hold its shape more neatly, with less juice escaping as you slice.