Cut the cod into chunks. Melt the butter in a non-stick pan and gently cook the cod until it starts to flake, about 5 mins. Flake the flesh into a bowl when it’s cool enough, then season. If the potato is cold, mash it well before adding to the fish, along with the peas, mint and mayo or yogurt. Carefully mix everything together. Flour your hands, then shape the mixture into eight patties. Dust both sides in flour, then chill for at least 30 mins.