Combine the meatballs, onion, brown sugar, soup mix and beer in a 3-1/2 quart crockpot. Cover and cook on LO for 3-1/2 to 4-1/2 hours or until heated through.
To make the sandwiches: Place 6 meatballs on each bun bottom. Sprinkle each sandwich with 1/4 cup cheese. Place on baking sheets. Broil 4-6 inches from the heat for 2-3 minutes or until the cheese is melted. Cover each with bun top.