Heat the stock to simmering point, and add all the ingredients except the fish and return it to the boil. Add the fish and simmer until the fish is cooked through. This dish can be eaten as a soup course, but as I have remarked before in Thailand soups are normally eaten with the other dishes of the dinner, rather than before them. Therefore you should use a slotted spoon to remove the fish and serve it in individual bowls to the diners, the soup liquor is then placed in a large serving bowl, from where they can help themselves (you can use a fire pot of fondue cooker to keep it hot if you wish).