Place half the sliced onion in the bottom of the crockpot. Add pork roast and water. Add remaining sliced onion. Cover and cook 8 to 12 hours on LOW. Remove bone and fat from pot, discard onions and water from pot. Shred the meat and put back in pot. Add the chopped onion and barbecue sauce to pot and cook for 2 hours on LOW, stirring frequently to avoid scorching.