Pour the coconut milk and measured water into a saucepan and bring to a boil.
Add the galangal, lemongrass, lime leaves, chillies, coriander roots or stalks and salt. Leave to bubble away over a medium heat for two minutes, until the flavours come through, then add the mushrooms and the tomatoes and cook for a further
two minutes, until the mushrooms are cooked. Remove from the heat and stir in the lime juice, soy sauce and coriander leaves. Serve immediately.