Melt the butter in a large pot over medium heat.
Add the sliced mushrooms and chopped onion and cook about 4 minutes, or until the mushrooms are tender and the onion is beginning to turn translucent. Season with salt and pepper.
Add ground beef and cook about 5 minutes, breaking up the meat as it cooks.
When the meat is no longer pink, drain off excess grease and then add the garlic and cook for another minute or two.
Stir in the flour and cook for 1 minute.
Stir in the beef broth, dijon mustard, Worcestershire sauce, and uncooked egg noodles.
Bring to a boil, then cover and simmer for about 10 minutes, or until the pasta is cooked. Season with salt and pepper, to taste.
Stir in the sour cream and cook just until heated through, about 2 minutes.
Spoon into bowls and top with parsley and/or freshly grated Parmesan, if desired.
Enjoy!