Cut beetroot into slices that are not too thick (wear kitchen gloves if possible!) And place in a bowl.
Put the vinegar, oil and onion salt in a small saucepan, bring to the boil briefly and pour everything warm over the beetroot. Cover and let stand for about 30 minutes.
Cut the onions into rings. Remove the beetroot from the marinade and drain.
Brush the plates with oil and sprinkle with a little salt. Arrange the beetroot slices in a circle on the plate.
For the dressing, mix together vinegar, oil, fish stock, horseradish and caraway seeds. Finally add the chives, season to taste and, if necessary, season again.
Arrange the trout fillets and onion rings on the beetroot salad. Spread the horseradish sauce over the salad. Garnish with onion sprouts or the sprout mix.