Boil peeled and chopped floury potatoes until tender, then drain. Put whole milk in a frying pan. Bring to a simmer, then add white fish, skin-side down. Gently cook until the fish is translucent and just flaking. Remove the fish from the milk, then discard the skin and milk. Break into large flakes.
Mash the potatoes, stir in wholegrain mustard, mayonnaise or tartare sauce, spring onions, zest of 1 lemon, a handful of chopped parsley and fish, then season well.
Form into 8 large patties. Put seasoned plain flour, a beaten egg and breadcrumbs on 3 plates. dip each cake into the flour, egg and breadcrumbs. cover and freeze.