Slice the bell peppers in half and remove the membranes. Preheat the oven to 400 degrees.
In a medium sized skillet on medium heat add the butter, oil, minced garlic, and salt and pepper. Saute until the onions and mushrooms are caramelized about 5-10 minutes.
Slice the roast beef into thin strips and and add them to the onion mixture. Cook for an additional 5 minutes until it is heated throughout.
Line the bottom of each pepper with a slice of provolone cheese. Fill the pepper until it is nice and overflowing. Add another slice of cheese on top.
Bake for 15-20 minutes or until the cheese is nice and bubbly and the pepper starts to soften.