Finely puree trout fillets, crème fraîche and horseradish with the chopping stick of the hand mixer. Whip the cream until stiff and fold into the trout mousse.
Clean and wash spring onions and cut into fine rings. Mix vinegar and 2 tablespoons of water. Fold in the oil and mix with the spring onions. Wash the lettuce leaves, drain and pluck into bite-sized pieces.
Cut the cucumber lengthwise into thin strips with the peeler. Mix the vinaigrette with the salad and the cucumber.
Wash the chives, shake dry and cut into fine rolls. Form trout mousse with 2 tablespoons into dumplings and arrange on the salad. Sprinkle with chives and coarse pepper.