Wash the potatoes thoroughly and cook them with their skins in boiling water for about 20 minutes. Drain, quench and let cool.
Peel the cucumber for the dressing. Cut about half of the cucumber into coarse pieces and put them into a tall mixing bowl. Add vinegar and mustard and puree everything finely with a hand blender. Stir in kefir well. Season to taste with salt, pepper and sugar. Wash the chives, shake dry and cut into fine rolls. Stir in, chill dressing.
For the tzatziki, peel and finely chop the garlic. Halve the remaining cucumber lengthwise and remove seeds. Grate the cucumber. Mix cottage cheese, cucumber and garlic, season with salt and pepper, chill.
Sort the spinach, wash and drain. Cut potatoes into slices and mix with dressing. Fold 50 g spinach into half the potato salad. Take the rest separately the next day. Eat tzatziki with it.