Preheat your oven to 375°F (190°C). Lightly grease a standard loaf pan or line a baking sheet with parchment paper. In a small skillet, sauté the diced onion in 1 teaspoon of olive oil over medium heat until softened, about 5 minutes. Add the minced garlic during the last minute. Allow to cool slightly.

In a large mixing bowl, combine the Panko breadcrumbs, milk, and beaten egg. Let stand for 3 to 5 minutes to allow breadcrumbs to hydrate.

Stir the sautéed onion and garlic mixture into the bowl, followed by the 1/2 cup grated Parmesan cheese, Italian seasoning, dried basil, oregano, salt, pepper, and fresh parsley. Mix well until evenly distributed.

Add the ground chicken to the mixture. Using clean hands, mix gently just until combined and large clumps are broken down. Do not overmix.

Transfer the mixture into the prepared loaf pan or onto the baking sheet, forming a uniform loaf shape. Brush the top lightly with a tablespoon of the marinara sauce.

Bake in the preheated oven for 45-55 minutes (free-form on sheet) or 55-65 minutes (in loaf pan). About 10 minutes before the end, spread the remaining marinara sauce over the top and sprinkle with mozzarella cheese and extra Parmesan. Continue baking until internal temperature reaches 165°F and cheese is melted and bubbly, about 5-10 more minutes.

Remove from oven and let rest, covered loosely with foil, for at least 10 minutes. Slice and serve.