In a skillet, brown steak in oil; transfer to a slow cooker. In the same skillet, saute onion for 1 minute. Gradually add water, stirring to loosen browned bits from pan. Add remaining ingredients; bring to a boil. Pour over the flank steak. Cover and cook on Low for 7-8 hours or until the meat is tender. Slice the meat; serve with onion and pan juices.