Melt 2 tablespoons butter in a large stockpot over medium-high heat. Add the steak and season with a few generous pinches of salt and pepper. Sauté for 5-7 minutes, stirring once every minute or so, until browned. Remove steak from the pot with a slotted spoon and transfer to a separate plate. Set aside.
Add the remaining 1 tablespoon butter to the stockpot, along with the mushrooms, onion and garlic. Sauté for 5-6 minutes, or until the onion is soft and the mushrooms are browned. Add in the white wine, and deglaze the pan by using a wooden spoon to scrape those tasty brown bits off the bottom of the pan. Let the mixture continue cooking for 3 minutes, until the liquid has reduced by about half.
Add in 5 1/2 cups beef stock, Worcestershire sauce, and egg noodles, and stir until combined. Continue cooking until the mixture reaches a simmer. Then reduce heat to medium-low and continue to simmer until the noodles are al dente, about 5-8 minutes, stirring occasionally.
Meanwhile, as the noodles are cooking, whisk the remaining 1/2 cup of beef stock and flour together in a small bowl until combined. When the noodles are al dente, stir the flour mixture and sour cream into the soup until they are combined. Continue simmering for 5 minutes. Stir the cooked steak back into the soup. Taste, and season with salt and pepper as needed.
Serve hot, garnished with parsley if desired.