How to make it
Preheat oven to 400°F.
Pour milk over the bread in a bowl so bread absorbs the milk. In a separate bowl, mix onion, egg, beef, parsley, cheese, garlic, sriracha, salt, and Worcestershire sauce together. Gently fold in the bread and milk mixture. Don’t overmix.
Divide the mixture into four parts (¼ pound each) and form each one into a meatball. Depress a cube of mozzarella into the center of each meatball, then cover up.
Space meatballs on a greased baking sheet so they do not touch. Bake meatballs for 30 minutes.
While meatballs are cooking, simmer tomato sauce. Once meatballs are cooked, place in simmering tomato sauce for 10 minutes. Add a little extra cheese to the top. Serve.