Wash the potatoes, possibly scrub the skin with a sponge. Cook the potatoes in boiling salted water for about 20 minutes until soft. Mix vinegar and mustard. Add oil and season with salt, pepper and some sugar.
Drain the potatoes and cut them in half lengthwise. Mix vinaigrette and potatoes in a bowl and let it stand for about 1 hour, stirring occasionally.
Wash, clean and quarter radishes. Cook the sugar snap peas in salted water for about 2 minutes. Drain and rinse under cold water. Drain the sugar snap peas. Roast sunflower seeds in a pan without fat.
Take out and let it cool down.
Wash the dill, shake dry and cut the flags into small pieces. Stir yoghurt, milk, lemon juice and horseradish until smooth, season with salt, pepper and a little sugar. Add 2/3 of the dill.
Add the yoghurt sauce, sunflower seeds, radishes and mangetouts to the potatoes and mix well. Season salad again and arrange in a bowl. Sprinkle with remaining dill.