While peppers roast, heat the olive oil in a large skillet over medium-high heat. When the oil is shimmering, add the steak and cook until browned on both sides, 6 to 7 minutes. Using tongs, transfer to a plate. Immediately add the garlic, onion, and mushrooms to the skillet and season with salt and black pepper. Cook for 5 to 7 minutes, until the onions are tender. Return the beef to the skillet. Stir to combine.