Saute about 5-8 minutes until starting to become translucent.
Thinly slice the mushrooms and add to the pan. Saute another 5-8 minutes.
Slice the roast beef into strips and add to pan. Cook another 2-3 minutes or until hot (assuming the meat is pre-cooked... if not, cook meat first until done).
Place the peppers in a large baking dish, open side up. Spoon the meat/onion/mushroom mixture evenly into the bell peppers. Top each pepper with the cheese and bake for at least 20 minutes or until well cooked. Enjoy!
Recipe by Wellness Mama® at https://wellnessmama.com/8134/philly-cheese-steak-peppers/