Take a large heavy pot and place beef roast, fatty side down. Pour vinegar over the beef roast and cover pot. Place in the refridgerator overnight. Next morning, drain off half of the vinegar and add 1 C. Water. Add sliced onions over the top of the roast. Slow cook roast until tender.
Take the beef roast and place in slow cooker fatty side down. Add vinegar to cover most of the meat, add onions and turn to high. Cook until tender.