Prepare the Peppers: Rinse bell peppers, cut in half lengthwise, and remove seeds. Toast them lightly in a pre heated oven for 5–7 minutes till it softens.
Cook the Vegetables: Heat oil in a pan and saute onions and mushrooms until golden and soft.
Add the Steak: Thinly slice the beef and add to the skillet. Add garlic powder, salt and pepper. Cook until steak is browned and juicy.
Stuff the Peppers: Fill each pepper half with the beef and veggie mixture (try to divide it among the peppers as evenly as possible).
Top with Cheese: Sprinkle shredded provolone or mozzarella generously over each stuffed pepper.
Bake: Arrange peppers in a baking dish, cover with foil, and bake at 375 degrees F for 15-20 minutes, or until cheese is melted. Rest 5 minutes before serving.
