COMBINE meat, dry soup mix and egg. Shape into 1-inch balls; place in slow cooker. Top with onions, peppers and barbecue sauce; cover with lid.
COOK on LOW for 5 to 7 hours (or on HIGH for 2 to 3 hours). Gently stir to evenly coat meatballs with the sauce.
PLACE 1 bun half on each serving plate; top with 1/4 of the meatballs.