Skin the brill fillets and trim any frill and a little off the tails to give 100 g/4 oz trimmings for the mousseline.
To make the mousseline, pour boiling water over the pistachios, leave for 30 seconds, then drain. Pull off the skins. Purée the fish trimmings with the pistachios, chervil and egg white in a processor or blender until smooth. Rub through a fine sieve. Season vigorously w