Cut the mushrooms into thin slices. Wash the radishes and also cut into thin slices. Put everything together with the chives and onions in a bowl.
For the dressing, whisk everything well, add to the salad and mix well. Let the salad stand for about 15 minutes.
Arrange the salad in the middle of 2 plates and drape the smoked trout on top. Coarsely grind the 3-colored pepper over the trout and sprinkle with the cress.
Tip: the 3-colored peppercorn mix is available in well-stocked supermarkets.