Serve this soup Asian-style. Slurp the noodles from the broth with chopsticks, then use an Asian soup spoon to scoop up what’s left. This simple soup comes together in less than 30 minutes.
Break the noodles in half and cook them in a large saucepan according to package directions until al dente, then drain. Rinse briefly with cool water.
Meanwhile, combine the stock, mushrooms, soy sauce, vinegar, sugar, and ginger in a soup pot. Bring to a rapid simmer, then lower the heat. Cover and simmer gently for 10 minutes.
Stir in the scallions, tofu, spinach, and optional bok choy. Cover and cook until the spinach is wilted but still bright green, about 3 minutes.
Stir in the noodles. Add two cups additional water, or enough to give the soup a dense but not overly crowded consistency. Season with pepper and some additional soy sauce if needed. Heat through and serve at once.