Place beef in slow cooker, and cover with onions. In a medium bowl, stir together ketchup, Worcestershire sauce, brown sugar, salt, paprika, mustard, and 1 cup water. Pour mixture over beef and onions.
Cover, and cook on Low for 9 to 10 hours, or until meat is tender.
Mix 1/4 cup water with flour to form a paste, and stir into goulash. Cook on High for 10 to 15 minutes, or until sauce thickens.