Put all the ingredients for the stock in a large saucepan with the water – there's no need to peel or chop the ginger and spring onions as they will be discarded after cooking. Bring to the boil, skimming off any scum, then cover with a lid and simmer for 1 hour. Strain the stock into a bowl and discard the flavourings.
While the stock is simmering, rinse the dried mushrooms in cold water and put them in a bowl. Pour over boiling water to cover and leave to soak for 20 minutes. Drain, then discard the tough stalks and cut the mushroom caps into thin slices.
Heat the sesame oil in a large saucepan, add the ginger and cook for 30 seconds, stirring. Add the prawn stock and soaked mushrooms, bring to the boil and simmer for 3 minutes. Add the spring onions, bamboo shoots, prawns, and soy and fish sauces. Bring back to the boil, then add the noodles and simmer for 2–3 minutes or until softened. Serve hot, sprinkled with coriander leaves.