Wash the mussels thoroughly, discarding any that aren’t tightly closed.
Trim and finely slice the spring onions, peel and finely slice the garlic. Pick and set aside the coriander leaves, then finely chop the stalks. Cut the lemongrass into 4 pieces, then finely slice the chilli.
In a wide saucepan, heat a little groundnut oil and soften the spring onion, garlic, coriander stalks, lemongrass and most of the red chilli for around 5 minutes.
Add the coconut milk and fish sauce and bring to the boil, then add the mussels and cover the pan.
Steam the mussels for 5 minutes, or until they've all opened and are cooked. Discard any unopened mussels.
Finish with a squeeze of lime juice, then sprinkle with coriander leaves and the remaining chilli to serve.