Cut fat from beef. Cut beef to fit into a 4-quart slow-cooker. Place beef in cooker. Top w/ onions. In bowl, combine tomato sauce, water, mustard, horseradish sauce, salt & pepper. Pour mix over beef and onions. Cover, cook on low for 8 hrs or on high for 5 hrs. Transfer beef and onions to platter. Transfer juices from cooker to a saucepan. Skim off fat. In small bowl, mix together the flour and water. Stir flour mix into sauce pan. Cook, stirring over medium heat until thickened. Cook 1 min more. Serve sauce spooned over beef and onions.