Prepare the herbs: remove any tough outer leaves from the lemongrass, trim, and chop the white, tender part. Peel and finely chop the garlic, peel the ginger and cut into fine shards.
Oil a wok or large pan and cook the lemongrass, kaffir lime leaves, garlic and ginger over a medium heat, keeping them moving so the garlic doesn’t burn.
Pour in the stock and coconut milk, bring to the boil, and boil for a few minutes while the chilli and chicken are prepared. If you are using chilli, halve and finely slice, either keeping or removing the seeds, dependent on taste. Chili flakes are also an alternative source of spice, if desired. Cut the chicken into bite size pieces.
Add the chicken and chilli to the soup, reduce the heat slightly and let the soup simmer until the chicken is cooked through. Depending on the size of the pieces, this could be between 8 and 15 minutes. If unsure, take out a piece and cut in half to check.
While the chicken is cooking, slice the spring onions, juice the limes, remove the coriander leaves from the stalks and chop roughly. When the chicken is cooked, add these to the soup, along with the sugar and fish sauce, seasoning to taste.