Place well-drained fish in a bowl and add potatoes. Beat egg yolk with lemon juice and add to fish with mustard, seasoning, dill, cornflour and nutmeg.
Heat oil until hot but not smoking. Add onion and sauté until pale gold, about 5 minutes, cool slightly and add to fish. Blend mixture lightly, using a sturdy fork.
Whisk egg white until fairly stiff peaks form and fold into mixture (this helps to lighten it).
Wet your hands and shape fish gently into cakes. Dust lightly with cornflour.
Heat a thin layer of oil in a frying pan until hot but not smoking. Add fish cakes, a few at a time, and fry on both sides until browned and crusty.
Serve with dill mayonnaise and a baby spinach, red onion and diced black olive salad.
DILL MAYONNAISE: Place egg yolks, lemon peel, garlic, mustard powder, lemon juice, vinegar, seasoning and dill in a medium-sized bowl. Whisk until frothy, using an electric hand beater or food processor fitted with the steel blade. still whisking, add oil drop by drop.
As mayonnaise begins to thicken, add oil in a slow but steady stream. If mayonnaise is too thick, add a little milk to thin it down.
Taste it - if it needs sharpening, add a little vinegar or lemon juice and adjust seasoning, if necessary.