Preheat oven to 350°F.
In a large bowl, combine the ground beef, ground pork, beaten egg, cheese, parsley, salt, and pepper. Mix together until just combined.
In a medium bowl, combine the milk, breadcrumbs, garlic, and onion.
Add the milk mixture to the meat mixture being careful not to overwork ingredients.
Form into desired size meatballs, greasing hands before starting to keep the meat from sticking, and place on a parchment-lined baking sheet.
Press your thumb into the meatball to create a well. Place a cube of cheese into the well and mold the meat over the ball. Roll in the palms of your hands to form an even ball and place back on the parchment-lined baking pan. Repeat with the remaining ingredients.
Place the pan in the preheated oven and roast until the outsides are browned and they read 165°F on a meat thermometer (about 25 minutes).
Top with warm marinara sauce.