Heat butter in large pan to 350 F, place steak, onion and mushrooms in and let sizzle for 30 seconds.
Flip steak, let cook 20 seconds, separate slices and mix in onion/mushrooms.
Finish cooking steak, form into 8” pile and top with cheese. Melt thoroughly.
Mix truffle oil into mayonnaise and spread on roll, place steak/cheese mixture in roll and top with horseradish.
