1. Cut the chicken crossways into thin slices. Place in a non-metallic bowl. Very finely slice one of the lemon grass stalks and scatter over the chicken with the spring onions. Add the lime zest, fish sauce and 1 tsp sugar and mix together. Leave to marinate for 30-60 minutes.
2. Roughly chop the remaining lemon grass and 2 chillies and put in a pan with the stock, half the ginger, sliced, and all the coriander stems (chop the leaves and reserve). Bring to the boil. Simmer, covered, for 30-60 minutes, then strain.
3. Gently fry the onion, fennel seeds, garlic and remaining ginger, finely chopped, in the oil in a large saucepan until the onion is soft. Add the strained stock and simmer for 10 mins then add the coconut milk. When the mixture comes back to a simmer, add the chicken with all its marinade and half the chopped coriander leaves. Simmer gently for 6-7 minutes until the chicken is cooked through.
4. Add the lime juice, seasoning and more sugar to taste. Sprinkle with the rest of the coriander and the third chilli, finely sliced, if liked.