Swabian Potato Salad (Kartoffelsalat)

Method

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1. 1Scrub the potatoes and put them, unpeeled, in a pot with cold salted water to cover. Bring to a boil and cook for 20-25 minutes, until a knife slides in easily but they aren't falling apart.
2. 2Drain and peel the potatoes while still hot but cool enough to handle. Slice them about 3-4 mm thick into a large bowl.
3. 3While the potatoes cook, make the dressing: heat the stock in a small saucepan, add the chopped onion and simmer for 2-3 minutes.
4. 4Take it off the heat and stir in the vinegar, mustard, sugar, salt and pepper. It should taste distinctly sharp and well seasoned; the potatoes will absorb it.
5. 5Pour the hot dressing over the warm potatoes and gently fold with a spatula without breaking the slices. It will look soupy at first; that's normal.
6. 6Let the salad rest for 20-30 minutes, folding occasionally. The potatoes should soak up the liquid and turn slightly creamy and glossy on the surface.
7. 7Fold in the oil and adjust salt and vinegar to taste. Serve lukewarm or at room temperature, topped with chives and the cucumber if using.