Preheat oven to 400*F.
Heat the butter and oil in a frying pan over medium heat. Dice onions and mince garlic; add to the frying pan. Saute until onions and garlic are caramelized.
Slice roast beef into thin strips and add to the onions and garlic. Cook 5-10 minutes.
Slice peppers in half lengthwise, remove seeds and ribs. Line each pepper with a slice of provolone cheese.
Fill each pepper with meat mixture until almost overflowing, then cover with another slice of provolone cheese.
Bake for 15-20 minutes until cheese is melted and golden brown.
