Splash a teaspoon of olive oil in the pan and when hot, add the fillets of mackerel and cook until the skin is crispy.
Put the fresh horseradish in a bowl with a tsp of caster sugar and the crème fraiche. Mix together with a splash of cider vinegar. Set aside.
Peel and slice the kohlrabi, purple radish, red onion and russet apple. Roughly scatter the capers and chervil over the salad. Then season it with olive oil and a squeeze of lemon juice. Mix together thoroughly and place on the plate.
To serve: Put the fillets on top of the salad and place the fresh horseradish crème fraiche at the bottom of the plate.