Cook the noodles according to the package directions.
Meanwhile, heat 1 tablespoon of the oil in a large skillet over medium-high heat. Season the steak with ¼ teaspoon each salt and pepper.
In 2 batches, cook the steak until browned, about 1 minute per side. Transfer to a plate.
Return the skillet to medium heat and add the remaining tablespoon of oil. Add the onion and bell pepper (if desired); cook until softened, 6 to 8 minutes.
Add the mushrooms and beef and cook until warmed through, about 2 minutes.
In a small bowl, combine the sour cream and steak sauce and stir the mixture into the beef and mushrooms. Serve over the noodles.