Wash the potatoes thoroughly and cook in boiling water for about 20 minutes. Drain, quench and let cool. For the dressing, wash, clean and peel the cucumber. Cut about half of the cucumber into large pieces.
Puree cucumber, vinegar, mustard and kefir with a hand blender, season to taste with salt, pepper and sugar. Wash the chives, shake dry and cut into fine rolls. Stir the chives into the dressing, except for a little for garnishing, and chill the dressing.
Peel and finely chop the garlic. Grate the remaining cucumber. Mix cottage cheese, cucumber and garlic, season to taste with salt and pepper, chill. Cut potatoes into slices and mix with the dressing, season again with salt and pepper.
Sort the spinach, wash, dab dry and fold into the salad.
Serve the salad. Spread the cottage cheese mixture on top. Sprinkle with chives and pepper.