Bring a large pot of salted water to a boil and cook the egg noodles according to package directions. Drain them, return to the pot, and toss with 1 tablespoon of butter. Cover to keep warm.

Season the beef strips with salt and pepper. Melt 1 tablespoon of butter in a large skillet over medium-high heat. Add the beef in a single layer and cook until browned on both sides, about 3 minutes. Remove the beef to a plate.

Lower the heat to medium and add the remaining tablespoon of butter to the same skillet. Add the onion and mushrooms and cook until the onion softens and the mushrooms release their liquid, about 5 minutes.

Stir in the garlic and cook for 30 seconds. Pour in the beef broth, Worcestershire sauce, and Dijon mustard. Bring to a simmer and cook for 2 minutes, scraping up any browned bits from the bottom of the pan.

Reduce the heat to low. Stir in the sour cream until the sauce is smooth, then return the beef and any juices to the skillet. Heat through for 1 minute without letting the sauce boil. Taste and adjust salt and pepper if needed.

Divide the buttered egg noodles among four plates or bowls. Spoon the beef and sauce over the noodles and sprinkle with chopped parsley.