Preheat oven to 400. Slice peppers in half lengthwise, remove ribs and seeds. Spray with nonstick cooking spray, lay cut-side down on cookie sheet and roast in oven for 15 minutes.
Slice onions and mushrooms. Saute over medium heat with butter, olive oil, minced garlic and a little salt and pepper. Saute until onions and mushroom are nice and caramelized, about 20 minutes.
Slice roast beef into thin strips and add to the onion/mushroom mixture. Cook 5 minutes.
Line the inside of each pepper with a slice of provolone cheese.When the meat mixture is done, drain in a colander (lined with a paper towel) to get rid of excess oil and water. Fill each pepper with meat mixture. Top each pepper with another slice of provolone cheese.
Bake for 15-20 minutes until the cheese on top is golden brown.