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In a large bowl, mix together the potato, spring onions, horseradish and mackerel, then shape into 8 even-size cakes. Roll the fishcakes in the flour, shaking off any excess, then dip in the egg, followed by the breadcrumbs. Cover and chill until ready to cook. Can be prepared up to a day ahead, or frozen.
Gently grill or shallow-fry the fishcakes for 5-6 mins on each side until crunchy, golden brown and hot all the way through. Serve with salad and lemon wedges.