Place in slow cooker, combine the meatballs, onion, brown sugar, soup mix, and beer. Cover and cook on LOW for 3-1/2 to 4-1/2 hours or until heated through. Serve with toothpicks for an appetizer. For the sandwiches, place six meatballs on each bun bottom. Sprinkle each sandwich with 1/4 cup cheese. Place on baking sheets. Broil 4-6 from heat for 2-3 minutes or until cheese is melted. Replace bun tops.