Separate coriander stalks from leaves. Coarsely chop stalks and finely chop leaves. Chop half spring onions. Put chicken stock into a large saucepan. Add coriander stalks, chopped spring onions, ginger, garlic, lemon,grass, lime leaves and chilli. Bring to the boil, cover and simmer gently for 30 minutes. Pass soup through a sieve into a clean pan. Add coconut milk, lime juice and fish sauce.
Diagonally slice remaining spring onions and add to pan. Bring to the boil and simmer gently for 5 minutes. Stir in chipped coriander leaves, garnish with coriander leaves and serve.