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 and add beer (let evaporate) then cheese sauce. Heat thoroughly.
Split pretzel roll and add steak mixture, top with fried onions and serve warm.