Scrub the mussels in cold water, scrape off any barnacles and pull away the hairy beards that protrude from the shells. Discard any with broken shells or any open mussels that do not close when tapped sharply.
Pour the stock and coconut milk into a large saucepan and bring to the boil. Stir in the lime rind and juice, lemon grass, curry paste, chillies, coriander and spring onions. Season to taste with salt and pepper.
Add the mussels, cover and return to the boil. Cook for 3–4 minutes or until all the mussels have opened. Discard any shells that remain closed. Use a slotted spoon to divide the mussels between 4 serving bowls and keep warm until ready to serve.
Bring the liquid to a vigorous boil and boil rapidly for 5 minutes or until reduced. Strain through a fine sieve, then ladle over the mussels. Garnish with chopped red chilli, chopped coriander leaves and lemon grass stalks.