Slice beef into super thin slices no more than 2" long strips. I used London Broil because it's what was on sale, but use whatever cut you want. In a medium bowl whisk together Worchester Sauce, Garlic, Onion Powder, Ground Pepper & Vegetable Oil. Add meat and toss in marinade. Cover and let sit in refrigerator for at least an hour (the longer, the better).
Saute Onions in 3 T. Butter over medium high heat until translucent and caramelized.
Saute Mushrooms in 3 T. Butter over medium high heat until browned and soft.
Fry meat (marinade and all) in remaining 2 T. of Butter over medium high heat until no longer pink. (about 10-15 minutes)
Preheat oven to 400.
Place 6 Peppers in 9x13 baking dish, cut side up. Generously salt & pepper the inside of each pepper. Place a slice of Cheese in bottom on each pepper. Repeat with remaining 6 peppers in another 9x13 baking dish. (If all of your peppers will fit without crowding in one large baking dish you can combine.)
Mix Meat, Onions & Mushrooms together and stuff mixture into peppers. Add 1/2 C. Water to each dish and cover with foil.
Bake 40 minutes. Remove from oven and carefully remove foil. Add shredded cheese to top of each pepper.
Return to oven and bake, uncovered, for an additional 10 minutes.