Put 3 of the onions in a slow cooker and then top with the brisket and then the remaining one onion.
Mix barbecue saude and 1 cup of water and then pour over all. Cover and cook on low for 6-8 hours or until brisket is fork tender. Let stand for 15 minutes before slicing Skim fat and reserve for the onion sauce.
Combine beans, rosemary and 1 1/2 cups of the sauce (in crockpot) in a saucepan and heat through.
Slice brisket and top with sauce and serve with the beans.