Place half of the sliced onions in the crockpot. Place roast on top and add water. Place remaining sliced onions on top of roast. Cover and cook 8-10 hours on LOW. Remove roast, discard fat and bone and shred meat with fork. Discard onions and water from pot. Place the shredded meat, chopped onion and barbecue sauce into the pot. Cook an additional 2 hours on LOW, stirring occasionally to prevent scorching. Serve on buns.