Bring the coconut milk and water to the boil in a pan. Add the lemongrass and all the other ingredients up to and including the ginger. Reduce the heat, cover and simmer for approx. 10 mins. over a low heat.
Add the chicken, cover and allow to infuse for approx. 10 mins., remove the chicken and pull away the meat.
Return the mushrooms and chicken to the soup, simmer for approx. 5 mins. Remove the lemongrass and kaffir lime leaf, season the soup. Sprinkle with the coriander, serve with the lime.