In 3 or 3 1/2-quart slow cooker, combine meatballs and onion. Sprinkle brown sugar and soup mix over beef and onion. Pour beer over top.
Cover; cook on Low setting for 5 to 6 hours.
Just before serving, gently stir mixture to coat meatballs with sauce. Serve with toothpicks. Meatballs can be held on Low setting for up to 1 hour.