German Potato Salad

Method

1. 1Boil the potatoes whole and unpeeled in salted water until a knife slides in with slight resistance, 20 to 25 minutes. Cooking them in their skins keeps them from soaking up water and falling apart.
2. 2While they cook, finely dice the onion and bring it to a simmer in the vegetable stock. Let it bubble for 2 to 3 minutes so the onion softens and loses its raw bite. Three hundred millilitres for a kilo of potatoes looks like a lot of liquid at this stage; it is the amount the slices will drink.
3. 3Whisk the mustard, apple cider vinegar, sugar, salt, and black pepper into the hot stock. Cider vinegar is what Swabian cooks reach for, and it earns its place twice over: milder against plain potato than red wine vinegar, and it leaves the slices pale yellow rather than tinting them grey-pink. Taste the dressing on its own — it should be sharp enough to make you wince, because the potatoes will mute it.
4. 4Drain the potatoes and peel them while they are still too hot to hold comfortably, working with a fork and a knife. Slice them into rounds about 4 mm thick on a board, then tip the whole lot into a wide bowl.
5. 5Ladle about half the hot dressing over the warm slices and fold gently, then leave the bowl alone for 15 minutes. Pouring in stages is what separates a schlotzig salad from a wet one: each addition has time to be pulled into the starch instead of collecting under the potatoes.
6. 6Add the rest of the dressing, cover the bowl, and let the salad stand at room temperature for at least an hour, folding it over two or three times along the way. In that hour starch migrates out of the cut surfaces and thickens the stock into a loose, glossy sauce. Before it, the salad tastes thin and the liquid sits apart; after it, the two have become one thing.
7. 7Fold in the sunflower oil and most of the chives at the very end. Oil goes last because it coats the slices and stops them taking up anything more — add it early and the salad stays watery. Taste, loosen with a splash of warm stock if it looks dry, and serve lukewarm with the remaining chives scattered over.