Wash and clean the mussels. Sort out open mussels. Peel the lemon grass and chop the soft interior finely. Clean the chilli, cut lengthwise, wash and remove the seeds. Chop the pod finely.
Bring coconut milk, lemongrass and chili to the boil in a large pot. Season with curry paste, soy and fish sauce. Add mussels. Cover and cook for 8-10 minutes, stirring occasionally.
Halve the peppers, clean, wash and cut into strips. Clean, wash and cut spring onions into rings. Put sprouts in a sieve, rinse and drain well. Lift the mussels out of the coconut milk with a skimmer, drain and put aside.
Add paprika and sprouts to the mussel stock, simmer for about 5 minutes. In the meantime, remove the mussel flesh from the shells, except for a few mussels for garnishing.
Add spring onions and mussels to the soup and cook for about 1 minute. Season soup to taste. Wash coriander, dab dry and remove leaves. Serve the soup, sprinkle with coriander.