This is a very filling sandwich, great to make ahead of time and keep warm in the oven for a football party or any other time.
In a large frying pan over high heat, add olive oil and heat so that a drop of water will sizzle when you drop it in the oil; lower heat to medium. Add onions and mushrooms, stir and cook until mushrooms darken and onions start to look transparent. Add steak slices and cook for approximately 3 minutes or until meat is lightly browned. Add salt and pepper to taste.
Heap cooked meat mixture in a long length-wise pile across the frying pan. Lay cheese slices over meat until melted. If using Cheese Whiz, melt in a double boiler or in the microwave.
Slice bread lengthwise. Using a spatula, scoop 1/2 the meat mixture and cheese and lay on bread with cheese on top. If using melted Cheese Whiz, ladle it on top. If you're using Cheez Whiz, do not use too much or it can overpower the sandwich's taste. Use remaing meat mixture to make another sandwich.
Slice sandwich into 2 or 4 pieces, and serve with a dill pickle.
Each sandwich makes 2 servings.
* Learn how to make Baked Caramelized Onions. This is a quick and easy way to caramelize a large amount of onions.
** Freeze steak before slicing. With a sharp knife, slice the meat paper thin.
Philadelphia Cheese Steak Sandwich Recipe: https://whatscookingamerica.net/History/Sandwiches/PhiladelphiaCheeseSteak.htm