Heat the meatballs and spaghetti sauce in a small saucepan. Allow to simmer on low until meatballs are heated through.
Place sandwich buns on a parchment-lined baking sheet.
Spread spaghetti sauce on both sides of each bun.
Place 3 meatballs on the bottom portion of each bun. Cut each slice of cheese in half and place a half on both the tops and bottoms of each bun.
Set oven to broil (550°F). Place baking rack in second space from the top. Broil for 4-5 minutes, or until cheese is melted and bun has browned on edges.
Remove from oven and let rest for 2-3 minutes before placing top bun over meatballs on bottom bun. Serve warm.
