First, soak the mung bean noodles in hot water for 25-30 minutes, and drain afterwards.
In a small pan, heat sesame oil, garlic, and red pepper flakes, until garlic is lightly brown. Mix it with shredded meat and set aside.
In a large saucepan, heat water and beef broth.
To the saucepan, add the bok choy, mushrooms, and the mung bean noodles.
Cook for 3-4 minutes until bok choy is softened and noodles are cooked.
Add the marinated meat and the scallions to the soup heat through.
The soup is ready to be served. Divide among bowls and serve with some extra Korean red pepper flakes and soy sauce.