Bring a large pot of salted water to a boil. Add the egg noodles and cook according to package directions. Drain, and transfer to a large bowl. Add the butter and toss until melted. Keep warm.
Meanwhile, season the steak with the salt and pepper. Heat 1 tablespoon of the oil in a large skillet over medium-high heat. Cook the steak, in two batches, until browned on both sides, about 4 minutes on each side. (The steak will naturally release its liquid before becoming golden brown.) Remove the steak to a plate, and keep warm, reserving the drippings in the skillet.
Add the remaining 1 tablespoon oil to the skillet. Add the onion and mushrooms and cook, stirring occasionally, until the onions and mushrooms are tender and golden brown, about 8 minutes.
Whisk together the stock, flour, Worcestershire sauce, and the mustard and pour into the skillet. Cook, stirring constantly, until the mixture thickens. Stir in the sour cream and the cooked steak.
To serve, divide the noodles evenly between serving bowls. Top with the stroganoff mixture and sprinkle with parsley.