Trim fat from meat. If necessary, cut meat to fit into a 3-1/2- or 4-quart slow cooker. Place meat in the cooker. Top with onions. In a small bowl, combine tomato sauce, the 1/4 cup water, the mustard, horseradish, salt, and pepper; pour over meat and onions.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours.
Transfer meat and onions to a serving platter. For sauce, transfer juices from cooker to a small saucepan. Skim off fat. In a small bowl, stir together flour and the 2 tablespoons cold water. Stir into the mixture in saucepan. Cook and stir over medium heat until thickened and bubbly. Cook and stir for 1 minute more. Serve with meat and onions. Makes 8 servings.