Wash the cucumber, dab dry and cut 3 l slices lengthwise from the middle (use the rest for other purposes). Crush trout fillet finely with a fork. Stir in horseradish. Season to taste with pepper and lemon juice. Spread the trout puree on the cucumber slices and roll them up.
Pin with wooden skewers. Cover the pumpernickel slices with some salad. Arrange the rolls on top. Garnish with tomato and chives