Heat grill to high. Lightly rub meat with oil. Grill on both sides until slightly charred and cooked to medium-rare, about 4 minutes per side. remove from grill and let rest for 5 minutes. Slice into 1/4." thick slices. Lay toasted hoagie halves on a flat surface and top each half with cheese slice. Divide the steak among 4 halves and cover with the other half. Wrap each sandwich in foil, place on the grill for 1 minute, each side. Remove from grill, unwrap foil and life top of bread to spread with horseradish mayonnaise on the inside of sandwich. Top with a few sprigs of watercress and replace top layer of bread. Cut sandwich in half at an angle and serve.