Prepare an outdoor grill with an oiled rack set 6 inches from the coals. On a gas grill, set the heat to medium.
Brush one side of steak with the mustard sauce. Grill and turn, adding more sauce. Grill to desired doneness: 14 minutes for rare (140 degrees), 20 minutes for medium (160 degrees), or 26 minutes (170 degrees) for well done. Slice thinly and serve.