If using raw steak (not leftovers), slice very thin. Season with salt and pepper.
Slice the pepper, onions and mushrooms. (1/4-1/8 - inch thick)
Heat olive oil in a skillet. Add the onions and cook for 1-2 minutes. Add peppers, mushrooms, salt and cook for 4-5 minutes, until softened and the liquid from the mushrooms has almost Â evaporated. Add the remaining seasonings.
Add the steak - if using raw shaved steak, cook for 2-3 minutes, if using cooked leftover steak, cook for 1 minute. Add the stock. Cook for 1-2 more minutes.
Preheat oven to 375 F.
Top with cheese and bake for 5-7 minutes, until the cheese is melted. Serve.