Wash the lemon hot, dry it: grate the rind finely and squeeze the lemon. Mix crème fraîche, horseradish, oil and lemon peel. Season dressing with salt, pepper, approx. 1/2 teaspoon sugar and lemon juice.
Clean the radishes, wash them and cut them into thin slices. Wash the cucumber, peel if necessary and cut lengthwise into thin slices with a wide peeler. Leek onions clean, wash, cut into very fine rings.
Spread the salmon slices on a plate. Mix the radish, cucumber and spring onions and place them in the middle of the salmon. Add dressing and sprinkle with poppy seeds. Serve with baguette.