You will need
Place the horseradish and vinegar in a bowl and set aside for 5 minutes to infuse, then squeeze dry.
Measure out 60ml/4tbsp of the cream and set aside. Place the remainder in a small bowl and whip until you have achieved soft peaks, then gently fold in the horseradish.
Chill until needed.
Place the apple in a pan with 30ml/2tbsp water and cook over a gentle heat for about 5 minutes until softend, stirring occasionally.
Beat to a puree with the small knob of butter and set aside.
Place the potatoes in a pan of boiling salted water, cover and simmer for 15-20 minutes until tender.
Drain and mash until smooth. Fold in the flour and egg yolks and then add the reserved 60ml/4tbsp cream, a little at a time until you have achieved a smooth batter. Season to taste.
Heat a flat griddle pan or large frying pan. Whisk the egg whites in a bowl until soft peaks have formed and carefully fold into the potato mixture.
Add the butter to the pan and then spoon on four mounds of the potato mixture. Cook gently for 3-4 minutes, then carefully turn over and cook for another 2-3 minutes or until puffed up and golden brown.
Transfer the potato pancakes to warmed serving plates. Divide the apple puree among them and arrange the trout fillets on top
Finish with a dollop of the chilled horseradish cream and garnish with the coriander to serve.