Preheat oven to 375°F and grease a baking sheet with olive oil.
In a large bowl, mix ground beef, ground pork, breadcrumbs, Parmesan, egg, milk, salt, pepper, garlic powder, and onion powder until combined.
Divide the mixture into 12 equal portions and flatten each piece.
Place a small amount of shredded mozzarella in the center of each portion and roll into a meatball, sealing the cheese inside.
Place meatballs on the baking sheet and bake for 18-20 minutes, or until golden brown and cooked through.
Warm the marinara sauce in a pan and pour over the baked meatballs.
Garnish with fresh parsley and serve warm.