Line a baking sheet with foil. Place frozen meatballs on the tray and bake according to package instructions.
Remove meatballs from the oven and switch the oven to broil. Top meatballs with a spoonful of sauce and about a teaspoon or so of shredded Mozzarella. Place the pan under the broiler for several minutes, watching closely, or until cheese is melted and bubbly.
Serve immediately, with toothpicks for easy eating.