Preheat oven to 375 degrees. Line a baking sheet with parchment paper or a silpat and set aside.
Tear slice of white bread into small pieces and place into a large bowl. Pour milk over top of the bread and let it soak while you chop the onions and garlic.
Add the remaining ingredients {excluding the cheese} into the bowl and mix thoroughly until all ingredients are evenly combined.
Scoop the meat mixture by the tablespoon into your hand and place a cube of cheese in the middle. Press the meat all around the cheese to cover completely and roll into a ball.
Place on prepared baking sheet and continue stuffing the meatballs with cheese until you have 24. Bake for 20-23 minutes or until meat is thoroughly cooked. Serve with marinara and spaghetti.