Pour a little of the coconut milk into a bowl. Add the curry paste and whisk until well combined. Add the rest of the coconut milk, a little at a time, then add the chicken stock, whisking until all the liquid has been incorporated into the mixture.
Heat a lidded pan, large enough to hold about twice the quantity of mussels you have, over a high heat.
Add the mussels, then the coconut milk mixture. Cover the pan with a lid and bring to the boil. Continue to steam the mussels for 2-3 minutes.
Add the chilli and garlic and stir the mussels carefully. Continue to boil for a further 1-2 minutes, or until most of the mussels have opened (discard any mussels that have not opened during cooking).
To serve, divide the mussels and cooking liquid equally among four serving bowls. Sprinkle over the chopped spring onions and coriander. Accompany with crusty bread.