Heat the oil in a large saucepan and cook the spring onions, galangal or ginger and chilli for 2 minutes until soft. Add the remaining ingredients except the mussels and warm gently until the sugar has dissolved. Turn up the heat and bring up to boiling point, then reduce the heat and simmer gently for 5 minutes to allow the flavours to develop.
Tip the mussels into the coconut sauce and cover with a tight-fitting lid. Cook for 3–4 minutes or until the mussels have opened – discard any that have not.
Spoon into serving bowls with plenty of the juices and sprinkle with extra coriander leaves and desiccated coconut, if using. Serve immediately with steamed jasmine rice or butternut squash.