In a large pot, heat the oil over a medium heat.
Saute the lemongrass and ginger until fragrant.
Stir in the coconut milk, water, fish sauce and sugar. Simmer for 10 minutes, to allow the flavours to infuse.
Add the mussels to the pot, and cover. Cook at a high heat for around 5 minutes, until all the mussels have opened up.
Discard any mussels that don’t open. Taste and adjust the seasoning with fish sauce.
Serve garnished with fresh coriander.