How to Make It
In a large pot, combine the beef stock with the water. Add the beef, onion, ginger, soy sauce, garlic and star anise and bring to a boil. Cover and simmer over very low heat until the meat is tender, about 2 hours. Transfer the meat to a plate and let cool slightly. Using 2 forks, coarsely shred the meat.
Strain the broth, discarding the solids. Return the strained broth and the meat to the pot. Add the shiitakes and the scallion whites and simmer until the shiitakes are tender, about 5 minutes.
Meanwhile, bring a large saucepan of salted water to a boil. Add the noodles and cook until al dente. Drain well and add the noodles to the soup. Transfer the soup to deep bowls, garnish with the scallion greens and serve, passing sesame or chile oil at the table.