Preheat oven to 400 degrees, and line a baking sheet with parchment paper greased with olive oil.
In a large bowl, combine beef, panko, egg, salt, pepper and parsley. Divide into 12 even portions. Press a mozzarella ball into each portion and form into a ball, making sure cheese is completely covered. Place meatballs on prepared baking sheet.
Bake meatballs for 10 to 12 minutes until beef reaches 150 degrees. Let rest, and serve with warmed tomato sauce.