Slice peppers in half lengthwise, remove ribs and seeds.
Slice onions, banana peppers and mushrooms. Saute over medium heat with butter, olive oil, minced garlic and a little salt and pepper. Saute until veggies are nice and caramelized, about 25-30 minutes. (If you don't like pepperoncinis you can leave these out)
Slice roast beef into thin strips and add to the onion/mushroom mixture. Allow to cook 5-10 minutes. If you use the frozen stuff like I did, just follow the cooking instructions on the package then add to onion/mushroom mixture when done.
Line the inside of each pepper with a slice of cheese. Then, fill each pepper with meat mixture until they are nearly overflowing. Top each with another slice of cheese.
Bake for 15-20 minutes until the cheese on top is golden brown.
You can drizzle a little pizza sauce over top if you like the extra zip.