Peel and de-vein the prawns. Wash the prawns, and the shells, very thoroughly. Put the prawns on a plate, sprinkle with 1/2 teaspoon salt and keep in the fridge until needed. Reserve the shells. Put the water into a large saucepan with the prawn shells and all the ingredients for the stock except the extra liquid. Bring this to the boil and simmer for 20-30 minutes. By this time the stock will be very fragrant from the lemon grass and other aromatic ingredients. Strain the stock through a fine sieve, or a sieve lined with muslin, into another large saucepan. Discard the solids. Add the extra stock or water, if used (this additional liquid is needed if the soup is to serve six people). Keep aside to reheat later. While the hot and sour stock is brewing, prepare the vegetables. Pick the watercress leaves off the stems as you wash them. Shred the oyster mushrooms finely. Put the clear, srained stock back on the heat; turn the heat up and bring the stock to a rolling boil, then add the prawns, mushrooms and coriander leaves. Let the mixture boil for 3 minutes, add the watercress and the fish sauce. Cook the soup for just one more minute, then serve immediately.