In a 3-qt. slow cooker, combine meatballs, onion, brown sugar, soup mix and beer. Cook, covered, on low until meatballs are heated through, 3-4 hours. , Place 5-6 meatballs on each bun bottom. Sprinkle each sandwich with 1/4 cup cheese. Place on baking sheets. Broil 4-6 in. from the heat until cheese is melted, 2-3 minutes. Replace bun tops.