Heat a soup pot over medium heat. Melt 2 tablespoons butter in the pot and add beef. Season to taste with salt and pepper. Cook, stirring frequently, just until steak is browned. Remove to a plate along with the juices.
Melt remaining butter in the now-empty pot. Add onion and cook until beginning to soften, about 5 minutes. Add mushrooms and cook until tender, about 5 more minutes. Season with salt and pepper to taste. Sprinkle in flour and cook for 2 minutes. Whisk in beef broth and cream. Bring to a boil. Add Worcestershire sauce and season to taste with salt and pepper.
Add noodles and cook for the length of time recommended on the package, or until cooked to your preferred firmness. Remove from heat. Stir in beef (along with the juices), sour cream, and parsley. Taste once more to adjust seasoning and serve.
• It’s easiest to slice beef thinly when it’s still somewhat frozen but not rock-hard.
• You can substitute ground beef for the steak if you prefer. If you do make that substitution, you can brown the meat “dry” in the pan and leave out the extra 2 tablespoons of butter.