Preheat oven to 375˚F. Spray a muffin pan with non-stick cooking spray.
Cut mozzarella into 1" cubes.
In a medium bowl, stir together ¼ cup breadcrumbs, onion, parsley, parmesan, garlic salt, pepper, garlic, and egg until well mixed.
Add the ground chicken and use your hands to gently mix together with the other ingredients (get your hands wet beforehand and it won't be as messy).
Form the meatballs by rolling the mixture into a ball with your hands, making a ball about the size of your palm.
Press a cube of cheese into the middle of the meatball and form meat around the cheese until covered.
Place remaining breadcrumbs into a bowl. Roll meatballs in the breadcrumbs, then place into muffin tin.
Bake for 20-25 mins, until cooked all the way through. Top with marinara sauce and serve!