Trim off any excess fat from the steak and cut it against the grain into very thin slices; set asi.Heat the oil in a large frying pan over medium-high heat until shimmering. Add the bell pepper, if using, and the onion. Season with salt and pepper and cook, stirring occasionally, until softened and starting to brown, about 5 minutes. If you’re using the mushrooms, push the peppers and onions to one side, add the mushrooms to the pan, and cook, stirring rarely, until browned, about 7 to 10 minutes. Stir to combine with the peppers and onions.Push all of the vegetables to one side of the pan, add the sliced steak in a single layer, and season with salt and pepper. Cook, stirring occasionally, until the meat is no longer pink, about 3 minutes. Reduce the heat to low.Divide the steak into 2 piles roughly the size of the hoagie rolls. Place half of the vegetables on top of each steak portion. Lay half of the cheese over each portion of steak and vegetables. Cover the pan with a tight fitting lid and let cook undisturbed until the cheese has melted, about 3 to 4 minutes. Using a flat spatula, scoop each pile into a roll and serve immediately.