Preheat oven to 400°F. Slice peppers in half lengthwise, remove ribs and seeds.
In a large sauté pan over medium-low heat, add the butter, olive oil, garlic, mushrooms onions and a little salt and pepper. Sauté until onions and mushroom are tender and they start to caramelize.
Slice the roast beef into thin strips and add to the onion and mushroom mixture. Allow to cook 5 to 10 minutes. Line the inside of each pepper with a slice of provolone cheese. Fill each pepper with the meat mixture until they are nearly overflowing.
Top each pepper with another slice of provolone cheese.
Bake for 15 to 20 minutes until the cheese on top is golden brown. (You may want to pre-bake the peppers for about 10 minutes while the meat mixture is cooking if you like a softer pepper)