Peel the skin of the lime. Put this in a saucepan together with the lemongrass, chilli pasta and ginger. Pour over the fish stock and bring to the boil. Simmer for 10-15 minutes then pour it into another pan through a sieve. Discard the debris.
Reheat the filtered soup. Add the green onions and cook for 2-3 minutes.
Taste it and, if necessary, add more chilli paste, salt & pepper. Add the seafood and cook for 1-2 minutes (under cover).
Sprinkle with coriander leaves and chilli slices just before serving.