Trim all excess fat from roast. Place potatoes and onion in bottom of crockery pot. Make a smooth paste of flour, mustard, chili sauce, Worcestershire sauce, vinegar, and sugar. Spread over top of roast. (Cut roast in half if necessary, to fit easily). Place roast in Crockery pot on top of potatoes and onions. Cover and cook on low setting for 10-12 hours (high 5-6).