Cook the noodles according to the package directions.
Meanwhile, in a large saucepan, over medium heat, heat the oil. Add the onion and salt and cook for 5 minutes. Add the mushrooms and bell pepper (if using) and cook for 5 minutes. Add the beef and cook until warmed through, about 8 minutes.
In a small bowl, combine the sour cream and steak sauce and stir the mixture into the beef and mushrooms. Divide the drained noodles among individual plates and top with the stroganoff.