3 lb Beef round steak; 1/2 inch -thick 1/2 c Flour 2 ts Salt 1/8 ts Pepper 1/2 ts Dry mustard 2 md Onions; thinly sliced & -separated in rings 2 cn (4 oz. each) sliced -mushrooms or; drained 1/2 lb Mushrooms; sliced 1 cn (10 1/2 oz.) condensed beef -broth 1/4 c Dry white wine; (optional) 1 1/2 c Sour cream 1/4 c Flour