Slash the fat edges of the steak. Use meat tenderizer, according to label directions. Broil on grill, about 2 inches from coals, a total of 25 - 30 minutes, turning frequently, do not overcook.

Blend butter, mustard and horseradish. Cut the bread lengthwise in half, toast both sides on the grill and keep hot in foil while you carve the steak. Slice meat with sharp knife, diagonally across the grain, at about a 30º angle, making very thin slices. Spread cut sides of bread, both halves generously with the mustard butter. Overlap meat slices across bottom half of loaf, making double or triple layers. Sprinkle with coarse pepper. Drizzle meat juices from carving platter over the steak slices. Add top half of loaf, cut on the bias into 8 sandwiches.