Coat the bottom of a skillet with extra virgin olive oil. Saute the onions, garlic, & mushrooms (if using) in the oil for 7-9 minutes on medium heat or until lightly browned & tender. Add the sliced roast beef strips & steak seasoning. Stir, & cook until warmed through.
Place a piece of cheese in the bottom of each pepper. Layer with the roast beef mixture, filling each pepper to about halfway. Then top the meat with another piece of cheese, fill the pepper with more meat, and then top each stuffed bell pepper with a final piece of cheese.
Bake at 400 degrees for 15-20 minutes, or until peppers are cooked through.