Wonderfully fragrant lemongrass stalks and lime leaves add plenty of flavour to this soup, but be sure to remove them before serving. Coriander stems with roots attached give a better flavour (wash them well), but lots of coriander is sold without roots - so just add a couple more stems. Bird's-eye chilies are available in many stores, but use 1 or more teaspoons of sambal oelek if unavailable. The dish pairs well with a tofu scramble with some Thai seasonings.