Preheat your oven to 200°C (400°F) and line a baking sheet with aluminum foil.

In a medium skillet, heat 1 tablespoon of olive oil over medium heat. Add the finely diced onion and minced garlic, sauté for about 5 minutes until softened and fragrant. Set aside to cool.

In a large mixing bowl, combine the ground chicken, cooled onion and garlic, seasoned breadcrumbs, milk, grated Parmesan, egg, chopped parsley, Italian seasoning, and salt. Mix gently until just combined.

On your prepared baking sheet, shape two-thirds of the meat mixture into a rectangle measuring about 10x6 inches. Create a small indentation in the center and layer the mozzarella cheese slices inside. Layer the remaining meat mixture on top and seal the edges to enclose the cheese.

Spread marinara sauce over the top of the assembled meatloaf. Bake for 45 minutes.

Combine panko breadcrumbs, additional grated Parmesan, melted butter, and chopped parsley in a small bowl. Stir until evenly coated.

After 45 minutes, sprinkle shredded mozzarella and distribute the breadcrumb mixture on top. Return to the oven for an additional 15–20 minutes or until the topping is golden brown and the internal temperature reaches 74°C (165°F).

Remove from oven and let rest for 10 minutes before slicing.