Preheat the oven to 200°C (180°C Fan/Gas Mark 6).
Place the potatoes into a saucepan, cover with cold water, and bring to a boil. Reduce to a simmer and cook for 15–20 minutes until tender.
Drain well and return the potatoes to the pan. Add the butter, mayonnaise, lemon zest, spring onions, salt, and black pepper. Mash until smooth.
Stir the tuna into the mashed potatoes until evenly combined.
Blitz the bread in a food processor until fine breadcrumbs form.
Place the flour, beaten egg, and breadcrumbs into three separate shallow bowls.
Divide the mixture into six equal portions and shape into fishcakes.
Coat each fishcake in flour, dip into the beaten egg, then coat evenly with breadcrumbs.
Arrange on a lined baking tray.
Bake the Tuna Fishcakes Recipe for 15 minutes, or until crisp and lightly golden.
Serve immediately with salad, steamed vegetables, tartar sauce, or lemon wedges.