Peel and chop the potatoes into even-sized chunks. Put them in a saucepan and just cover with boiling water. Add a pinch of salt, bring back to the boil and simmer for 10 mins
Place the fish in the oven Gas Mark 5 for about 10 mins. Remove from the oven and leave to cool. Meanwhile drain the potatoes, leave for a minute or two before adding the lemon juice, parsley, spring onion and mustard.
Flake the fish in big chunks into the potatoes and add the egg yolk. With your hands mix together lightly as you don‘t want the fish to break up. Carefully shape them into 8 fish cakes. Place onto a tray and put in the fridge for 20 mins.
Beat the egg on a large plate. Spread the breadcrumbs on a baking sheet. One by one, brush a little flour over the cakes, sit each cake in the egg, and brush over the top and sides so it is completely coated. Sit the cakes on the crumbs, patting the crumbs on the sides and tops so they are lightly covered.
Heat the oil in a large frying pan. To test when ready, drop a piece of the dry breadcrumbs in - if it sizzles and quickly turns golden brown, it is ready to use. Fry the fish cakes over a medium heat for about 5 mins on each side or until crisp and golden
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