Poach the fish gently in salted water for 3–4 minutes, then drain and allow to cool. Flake the fish, checking for any small pin bones as you do so. In a bowl, mix half of the fish with the potato, anchovy essence, mustard, parsley, chives and salt and pepper until smooth. Gently fold in the remaining fish.
Shape the mixture into 4 large round cakes or 8 small o