Melt butter in a large pot or skillet over medium heat. Sauté mushrooms and onion, stirring occasionally, until mushrooms are tender and browned, about 10 to 15 minutes. Onions should be golden brown and beginning to caramelize.
Add ground beef and break up the pieces as it browns, about 3 to 5 minutes. Stir in beef broth, Worcestershire sauce, Dijon mustard, garlic, thyme, salt, pepper, and egg noodles. Cover and bring to a boil; reduce heat and simmer until pasta is cooked through, about 10 to 15 minutes, stirring once or twice. If liquid begins to evaporate before the pasta is cooked, add more water or beef stock.
Stir in sour cream and cook another minute until heated through. Garnish with parsley (optional) and serve hot.
Per serving: 386 calories, 19 grams fat (10 grams saturated fat), 122 milligrams cholesterol, 877 milligrams sodium, 18 grams total carbohydrates, 1.5 grams fiber, 35 grams protein.