"This is a versatile dish: You can eat it as a main, make steak sandwiches with it, or top rice or noodles! (Edit: I added a cup or so of sour cream to this last night and made a fancy beef stroganoff, served over noodles...really great!)"
Slice the beef very thinly across the grain with a sharp knife.
In a bowl or freezer bag, combine the soy sauce, worcestershire, red wine vinegar, olive oil, garlic, sugar, and pepper. Add the beef and marinate for at least 1/2 hour or up to 4 hours.
In a large skillet, heat butter or oil over medium high heat, and saute the green pepper, onion, and mushrooms until tender and lightly browned, about 6 minutes. Remove from skillet and set aside.
In the same skillet, fry the marinated steak strips until desired doneness is reached, then return the onion/pepper/mushroom mix to the pan. Add the swiss cheese slices, and stir until melted. Season with salt and pepper to taste, and serve. Enjoy!