Remove the tough outer leaves from the lemon grass and chop the core into 2.5 cm (1 inch) pieces. Place in a large saucepan with the coconut milk, garlic, ginger, chilli, sugar, lime leaves and fish sauce.
Bring to the boil and reduce the heat to a simmer. Add the chicken, partially cover with a lid and cook gently for 6–7 minutes.
Stir in the lime juice and herbs and serve immediately.