Boil the water and add the shiitake mushrooms and let steep for 10 minutes. Then remove the mushrooms and roughly chop them, set aside. Pass the mushroom water through a sieve and collect in a saucepan. Add the broth to the mushroom water and bring to the boil. Add ginger, garlic, shiitake, mushrooms and soy sauce. Cover the soup and simmer on a low heat for 10 minutes.
Pour hot water over the pasta in a bowl and let it steep for 10 minutes. Drain and add to the soup. Let simmer for another 3 - 4 minutes. Scatter the spring onions over the soup and garnish with coriander.