Preheat oven to 400 degrees. Place bell pepper halves in a rectangular baking dish sprayed with non-stick cooking spray. Sprinkle pepper cavities with salt to taste. Bake for about 15 minutes. Keep oven heated to 400 degrees.
Add 2 tablespoons olive oil to a large sauté pan and heat on medium high. Add sliced onions and mushrooms and cook until liquid has released and cooked off and veggies are tender. Sprinkle with salt to taste. Remove vegetables from the pan and set aside.
Add another tablespoon of olive oil to the same pan. Add beef and cook until meat is browned. Season with salt to taste. Add veggies back to the pan with the meat and stir to blend ingredients.
Place 1 slice of Provolone cheese into the cavity of each pepper half. Scoop steak and veggie mixture into the cavity on top of the cheese. Place another slice of cheese over the steak. Bake Philly Cheesesteak Stuffed Peppers for about 20 minutes, or until cheese is melted.
Spicy Taco Stuffed Peppers - cooked ground beef in taco seasoning, stuffed in peppers, and baked until peppers are crisp tender, top with shredded lettuce and cheese, a dollop of sour cream, and sliced spring onions.
Meatloaf Stuffed Peppers - raw meatloaf stuffed in peppers and baked until meat is cooked through, top with a ketchup/brown sugar glaze and broil just until glaze is bubbly.
Chicken and Rice Stuffed Peppers - cooked rice mixed with shredded rotisserie chicken (or shredded baked chicken breast), stuffed in peppers and baked until peppers are crisp tender, then top with a cheese slice (optional).