Preheat your oven to 200°C (400°F). Line a baking sheet with foil and spray with cooking spray.
Heat the olive oil in a pan over medium heat. Cook the onion and garlic until softened, about 5 minutes. Let this cool completely.
In a large bowl, combine the ground chicken, cooled onion mixture, breadcrumbs, milk, Parmesan, egg, parsley, Italian seasoning, and salt. Mix just until combined – don’t overmix.
Pat two-thirds of the chicken mixture onto the prepared baking sheet, forming a rectangle about 20cm x 10cm. Arrange the mozzarella cheese string pieces down the centre, then top with the remaining chicken mixture. Shape into a proper loaf, sealing the cheese inside.
Spread the marinara sauce over the top of the meatloaf. Bake for 45 minutes.
Meanwhile, mix together the panko breadcrumbs, Parmesan, melted butter, and parsley for the topping.
After 45 minutes, remove the meatloaf from the oven. Top with the shredded mozzarella, then sprinkle over the breadcrumb mixture.
Return to the oven and bake for another 15-20 minutes until the topping is golden and the centre reaches 74°C (165°F).
Let the meatloaf rest for 10 minutes before slicing and serving.