Mix horseradish and yoghurt. Wash the apple, rub dry, quarter and core it. Cut into thin slices. Sprinkle with lemon juice
Spread slices of bread with half of the horseradish cream. Alternately cover with apple and ham slices. Spread the rest of the horseradish cream on top as a blob. Cut the cress from the bed and sprinkle over it. Garnish with chives