Combine beef, pork, crumbs, milk, egg, italian seasoning, and onion. Mix well and form into 1 1/2 inch meatballs. Cube the mozarella into 1/2" cubes.Press 1 cube into each meatball, cover well with the meat.Place on ungreased cookie sheet and bake for 18-20 minutes. In a saucepan heat the pizza sauce. Add meatballs and warm through. Slice a V shaped lengthwise cut into sub rolls. Use the cut out for breadcrumbs later. Pile down the cut 5-6 meatballs. Spoon on sauce and lay a slice of mozarella cut in half so it covers the length of the roll on the top of the sandwich. Place under the broiler for 2-3 minutes or till bubbly.