In cast iron skillet, sear shaved steak, to desired wellness, salt & pepper to taste. Remove from pan. Add butter to pan, melt. Add onions, saute 3 min, add mushrooms, saute 1 min more (cook onions and mushrooms longer or shorter, if desired). Add steak to pan and saute 1 min more. Add steak, cheese, peppers, onion and mushroom to Kaiser roll. Put sandwich in toaster oven and toast to desired crispiness. Top with southwest sauce and enjoy with your favorite side, chips, fries, etc!