Philly Cheesesteak Stuffed Peppers

I love, love, love the flavor of this dish.  The peppers are a nice sub for the heavy bread.  This is a lighter version of the popular dish, but is a must try.  Delicious!  Enjoy!
-2 Tbs butter
-2 Tbs extra virgin olive oil
-1 medium onion, halved and thinly sliced
-8 oz baby bella mushrooms, sliced
-2 cloves garlic, minced
-1 tsp salt
-1 tsp pepper
-7 oz deli sliced roast beef, cut into thin strips
-2 large bell peppers, halved and cleaned
-8 slices provolone cheese
Preheat oven to 400 F.  Lightly coat a baking dish with cooking spray and set aside.
Melt the butter in large skillet and add the oil.  Add the onions and mushrooms.  Saute for about 20 minutes, or until soft.  Add the garlic.  Season with salt and pepper, and cook for 2 more minutes.
Add the beef strips and cook for 7-8 minutes.
Place pepper halves into prepared baking dish.  Place 1 slice of  cheese inside each pepper.  Fill each pepper evenly with the meat mixture.  Cover each pepper with 1 more slice of cheese.
Bake in preheated oven for 15-20 minutes or until the cheese on top is golden brown.