Slice orange bell pepper in half lengthwise and remove seeds and pith. Repeat for red and yellow peppers.
Brush the outside of each bell pepper with olive oil.
Place pepper halves, cut side down, into the baking dish. Bake for 25-30 minutes.
Heat one tablespoon of olive oil in skillet over medium-high heat until shimmering.
Add sliced steak to the skillet and season with garlic salt, black pepper, and onion powder.
Add sliced green pepper, red onion, and sliced mushrooms. Cook, turning frequently, until no pink remains in meat and vegetables are tender crisp. Remove from heat.
Layer mozzarella cheese, meat and vegetable mixture, then more mozzarella cheese. Continue until all pepper halves are stuffed.
Bake for 6-10 minutes, or until cheese is melted and browned to your liking.
Thinly sliced ribeye is the traditional beef of choice for Philly cheesesteaks, so that is what we are using here. You can substitute with flank steak or top round. It's definitely doable to make these stuffed peppers with ground sirloin as well.
Either shredded mozzarella, provolone, or white American cheese will work on top of your peppers. Go with your favorite. We ended up doing a combination of mozzarella and provolone in ours.