Puree the onion, chillies, lemongrass, lemon juice, coriander powder and turmeric together in a blender.
Heat some olive oil in a saucepan and roast the paste in it for 5 minutes while stirring. Now pour in the fish stock and coconut milk, bring to the boil with the fish sauce.
Wash and add the mussels, then cook covered for about 8 minutes until the mussels have opened. Serve with fresh coriander.