Trim all excess fat from roast. Place potatoes and onion in bottom of crockpot.
Make a smooth paste of flour, mustard, chili sauce, worchestershire sauce, vinegar, and sugar. Spread over top of roast (cut roast in half, if necessary, to fit easily). Place roast in crockpot on top of potatoes and onions. Cover and cook on Low for 10 to 12 hours or on High for 5 to 6 hours.
Serves 4 to 6