Puree the onion, red chillies, lemongrass, lemon juice, coriander powder, and turmeric in a blender.
Heat a few tablespoons of extra virgin olive oil in a saucepan over medium-high heat. Add the blended paste and roast for 5 minutes, stirring constantly. Pour in the fish stock and coconut milk, add the fish sauce, and bring to a boil.
Wash the mussels and add them to the saucepan. Cover and cook for 8 minutes over medium heat until the mussels have opened. Serve garnished with fresh coriander.