Wash the mussels thoroughly and then discard any that aren’t properly closed.
Peel and slice the garlic cloves. Roughly chop the coriander and set aside. Trim and finely slice the spring onions. Cut the lemongrass into 4-6 pieces. Remove the stems from the chilli and then finely slice it.
In a large saucepan, heat some olive oil and then add the spring onion, garlic, lemongrass and most of the red chilli. Soften the ingredients for about 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. Discard any mussels which haven’t opened.
Transfer to a serving bowl and drizzle over the juice of the lime and sprinkle with the chopped coriander and the remaining chilli slices. Serve immediately.