Wash potatoes thoroughly, cut in half or quarters and cook in boiling salted water for about 20 minutes. Sort, wash and clean the spinach. Wash and clean radishes and cut into thin slices.
Peel, halve and finely dice the onion.
Mix onion, mustard, vinegar and 3 tablespoons of potato cooking water. Add oil and season to taste with salt, pepper and sugar. Drain potatoes. Mix spinach, radish, potatoes and dressing, arrange on plate, sprinkle with pepper and horseradish.