Skin the fish fillets, cut them into pieces and put them in a blender or food processor.
Peel off the thick outer leaf of the lemon grass stalk, top and tail it and cut across into the thinnest rings you can manage. Finely chop the coriander and chillies. Add all of these to the fish, together with the fish sauce and the egg. Blitz the mixture to a smooth purée, scrape this into a bowl,