Clean the mussels and dispose of broken and open specimens. Finely dice the garlic and ginger, core the chilli and cut into rings. Halve the lemongrass lengthways and cut into pieces about 3 cm long.
Bring the fish sauce to the boil with 125 ml of water in a large saucepan or wok, add the mussels and sprinkle with garlic, ginger, chilli and lemongrass. Cover and steam for about 5 minutes until the mussels open. Finally add the lime juice.