Thai herbs and spices give a delicious fragrant quality to soups, whether they are based on meat or fish. These days you can buy Thai spices quite readily from Asian greengrocers and many supermarkets.
Put all the ingredients for the stock into a pan with the stalks from the coriander for the soup and some salt and pepper. Bring to the boil and simmer gently for 1 hour, occasionally skimming off any scum.
About halfway through that cooking time, mix the cornflour with a little water and stir it into the stock. Simmer for 20 minutes, strain the stock and return t