preheat oven to 400

slice peppers in half length wise and remove ribs and seeds

in large saute pan over low medium heat add butter, olive oil, garlic, mushrooms, onions, and salt and pepper to taste. saute until onions and mushrooms are Caramelized. bout 30 min.

add sliced roast beef to mushroom/onion mix and allow to cook 5-10 min

line ea pepper with a slice of cheese. stuff with meat mixture until nearly overflowing and top with another slice of cheese

bake for 15-20 min until cheese on top is golden brown