Cook the noodles: Bring a pot of salted water to a boil. Cook egg noodles according to package instructions, drain, and set aside.
Brown the beef: In a large skillet, cook ground beef over medium heat until browned. Remove from pan and set aside, leaving some fat in the skillet.
Sauté vegetables: Add butter, onion, mushrooms, and garlic to the pan. Cook until softened and lightly browned.
Make the sauce base: Sprinkle flour over the veggies and stir well. Slowly whisk in beef broth until the mixture thickens slightly.
Add beef & flavorings: Stir in Worcestershire sauce, Dijon mustard, and the cooked ground beef. Simmer for about 5 minutes.
Finish with sour cream: Reduce heat to low, stir in sour cream, and mix until creamy. Do not boil after adding sour cream to prevent curdling.
Combine with noodles: Toss in the cooked noodles until coated in the sauce. Garnish with parsley and serve warm.