Halve the chilli and keep the seeds for extra heat, if liked. Put the chilli, shallots and lemon grass into a liquidizer goblet with the ginger and process together until finely chopped.
Heat the oil in large, deep saucepan, add the finely chopped ingredients and fry over a medium heat for 5 minutes, stirring until softened. Add the coconut milk, kaffir lime leaves, fish stock and fish sauce and cook for 3 minutes. Set aside until ready to finish.
Meanwhile, pick over the mussels and discard any that are opened or have cracked shells. Scrub with a small nailbrush, remove any barnacles and pull off the small, hairy beards. Put them in a bowl of clean water and leave until ready to cook.
Reheat the coconut milk mixture. Drain the mussels and add to the mixture. Cover the pan with a lid and cook for about 5 minutes until the mussel shells have opened.
Spoon the mussels and the coconut sauce into bowls, discarding any mussels that have not opened. Garnish with the coriander.