Bring a large pot of salted water to a boil and cook the egg noodles according to package directions. Drain and return them to the pot. Add 2 tablespoons of butter and toss until the noodles are coated. Set aside and cover to keep warm.

Heat a large skillet over medium-high heat. Add the remaining 2 tablespoons of butter. Season the beef with salt and pepper, then cook in batches until browned on both sides. Remove the beef to a plate and set aside.

In the same skillet, add the onion and mushrooms. Cook until the onion softens and the mushrooms release their liquid, about 5 minutes. Stir in the garlic and cook for 1 minute more.

Pour in the beef broth, Worcestershire sauce, and Dijon mustard. Scrape the bottom of the pan to loosen any browned bits. Let the mixture simmer for 3 to 4 minutes until it reduces slightly.

Lower the heat and stir in the sour cream until the sauce is smooth. Return the beef and any juices to the skillet. Stir gently and heat through for 2 minutes. Taste and adjust salt and pepper as needed.

Divide the buttered noodles among four plates. Spoon the beef and sauce over the noodles. Sprinkle with chopped parsley before serving.