In a 6-quart pot set over medium heat, add the ground beef, mushrooms (if using - see note), onion, and garlic. Cook for 5 to 7 minutes, breaking the meat into small pieces, until the meat is no longer pink. Drain excess grease.
Add the broth, bouillon paste (if using), and Worcestershire sauce. Bring to a boil over medium-high heat.
Add the noodles, reduce the heat to medium, and simmer the noodles until al dente according to package directions (they'll cook for a few more minutes in a later step). Partially cover the pot if the cooking time is more than 10 minutes.
In a blender, combine the milk, sour cream and flour and process until well-blended and smooth. Pour this mixture into the soup. Cook, stirring constantly, over medium heat until the soup comes to a simmer. Continue cooking for 3 to 4 minutes until the soup has thickened.
Season the soup to taste with salt and pepper (important!).
Serve, garnishing individual portions with fresh parsley, if desired. The soup will thicken considerably as it cools.
