For what we Sheffielders call a fishcake as opposed to the rissoles we have foisted on us elsewhere you require the following;

One large potato, cut into 1/4 inch slices
cod or haddock fillets around the same thickness as the potato
flour and water batter

Cut the fish to fit the potato slices and sandwich between. Holding the slices by finger and thumb dip them in the batter until fully coated and then carefully lower into hot oil. When fully cooked when they float to the surface.