Cook the Noodles: Bring a large pot of salted water to a boil. Cook the wide egg noodles according to the package instructions until al dente. Drain well, toss with a little butter if desired to prevent sticking, and set aside.

Brown the Beef: While the water is boiling, heat a large skillet over medium-high heat. Add the ground beef and chopped yellow onion. Cook for 5 to 6 minutes, breaking the meat apart with a wooden spoon, until the beef is fully browned and the onions have softened. Drain any excess grease from the pan.

Saute the Mushrooms: Reduce the heat to medium. Add the sliced mushrooms to the skillet with the beef and onions. Cook for 4 to 5 minutes until the mushrooms have released their moisture and softened. Stir in the minced garlic and cook for 1 additional minute until fragrant.

Create the Sauce Base: Push the meat and mushroom mixture to one side of the skillet. Melt the butter on the empty side of the pan. Once melted, sprinkle the all-purpose flour over the butter and stir to combine. Let the flour cook for 1 minute to remove the raw taste, then mix it thoroughly into the beef and mushrooms.

Simmer and Thicken: Slowly pour in the beef broth, scraping the bottom of the pan to release any browned bits. Stir in the Worcestershire sauce, Dijon mustard, salt, and black pepper. Bring the mixture to a gentle simmer and let it cook for 3 to 5 minutes until the sauce has beautifully thickened.

Add the Sour Cream: Remove the skillet entirely from the heat. This is crucial to prevent the dairy from curdling. Stir in the room-temperature sour cream until the sauce is smooth, creamy, and uniform in color. Taste and adjust the salt and pepper if necessary.

Serve: Spoon the warm, creamy ground beef stroganoff generously over the cooked egg noodles. Garnish with fresh chopped parsley and serve immediately.