Heat the oil in a large saucepan, add the shallot and cook for 2 minutes. Stir in the garlic, chilli and lime leaves and cook for 1 minute more. Pour in the coconut milk and the measurement water, add the lemon grass, fish sauce and sugar and leave to simmer for 10 minutes.
Add the mussels, cover and cook for 3–5 minutes until the mussels are open, discarding any that do not open. Scatter over the coriander to serve.