Heat a drizzle of oil in a large frying pan or wok on a medium heat.
Trim the lemongrass stalk and discard any tough outer leaves before finely chopping the white bit (discard the green bit as it can be quite bitter). Peel and finely chop the garlic and then peel the ginger and cut it into thin slivers. Tear the kaffir lime leaves in several places on the leaf (but keep the leaf intact.) Chop the stalks off the cilantro (in one go) and then finely slice them (keeping the cilantro leaves aside for later).
Carefully toss everything in the hot oil with the kaffir lime leaves and stir fry for a couple of minutes, being careful that nothing sticks and burns.
Next add the coconut milk and stock and bring to a simmer.
Meanwhile, halve the chili lengthwise and then finely slice it, leaving the seeds in if you like it quite fiery. Chop the chicken into bite-size pieces and add both ingredients to the now-simmering soup. Reduce the heat a little and leave it to bubble away for about 8 minutes until the chicken is cooked.
Finely slice the onions (both the green and the white bits), juice the limes and roughly chop half of the reserved cilantro leaves. Add these once the chicken is cooked and then leave to simmer for a final minute. Last, add enough fish sauce and sugar to taste.
Ladle into four serving bowls, scatter the remaining cilantro leaves over and serve.