Heat 2 cups water, and vinegar together until hot. Place meat in crock covered with vinegar and water. [Do NOT use a metal pot!] Add salt, parsley, pepper, sugar, and onion. Cover and refrigerate for 3 days, turning daily. Drain marinade and reserve. Lightly flour meat. Brown well in butter in kettle. Add onion slices and 3 cups marinade. Cover and simmer until tender. Remove meat to a hot serving dish and keep warm. Skim fat off liquid. Mix flour and water and stir into pan. Add stock,