1. Heat 1 tbsp of butter in a large skillet over medium-high heat. Season beef with salt and pepper, and cook until browned. Remove and set aside.
2. In the same skillet, add the remaining butter and sauté onions, garlic, and mushrooms until softened.
3. Sprinkle flour over the mushroom mixture and stir well. Gradually add beef broth, whisking to create a smooth sauce.
4. Stir in sour cream and Dijon mustard, mixing until the sauce is creamy and well combined.
5. Return the beef to the skillet and simmer for 5 minutes. Adjust seasoning if needed.
6. Serve hot over egg noodles, garnished with parsley.