Start by preparing the dressing; whisk together the honey, mustard and sherry vinegar with some seasoning in a small mixing bowl until smooth. Whisk the sunflower oil very gradually until you have a thickened dressing. Reserve to one side.
Flake the mackerel flesh away from the skin of the strips and reserve in a bowl. Toss the watercress with the dressing in a mixing bowl, using your hands to toss and combine evenly. Arrange on 4 serving plates and sprinkle the mackerel flesh evenly on top.
Garnish the perimeter of the salad with slices of the radish. Finish by sprinkling the roughly chopped walnuts on top.