Preheat your oven to 200°C (400°F) and prepare a baking sheet by lining it with foil and spraying it with cooking spray.

In a medium skillet, heat a tablespoon of olive oil over medium heat. Add the finely diced onion and minced garlic, sautéing until softened and fragrant, about 5 minutes.

In a large mixing bowl, combine the ground chicken with the cooled onion and garlic mixture. Add in the seasoned breadcrumbs, milk, grated Parmesan cheese, egg, chopped parsley, Italian seasoning, and salt. Gently mix until just combined.

Take about two-thirds of the chicken mixture and shape it into a rectangle, creating a well in the center. Place the mozzarella cheese slices into the well, then cover them with the remaining meat mixture.

Spread marinara sauce evenly over the top of your formed meatloaf, then pop it into the preheated oven and bake for approximately 45 minutes.

While the meatloaf is baking, prepare the topping by mixing panko breadcrumbs, remaining grated Parmesan cheese, melted butter, and chopped parsley.

After 45 minutes, remove the meatloaf from the oven, sprinkle with mozzarella cheese and the crispy topping mixture, then bake for an additional 15 to 20 minutes.

Once baked, let the meatloaf rest for about 10 minutes, then slice and serve.