Crockpot Beef Diablo

2 1/2 Pounds pot roast, boneless/beef arm
3 Potatoes, peeled & sliced
1 Onion, sliced
2 Tablespoons flour
1 Tablespoon prepared mustard
1 Tablespoon chili sauce
1 Tablespoon Worcestershire sauce
1 Teaspoon vinegar
1 Teaspoon sugar

Trim all excess fat from roast. Place potatoes and onion in bottom of crockpot; 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 crockpot on top of potatoes and onions, cover and cook on Low for 8 to 10 hours or on High for 4 to 5 hours.