The quintessential Viennese warm potato salad: firm waxy Kipfler potatoes sliced warm, steeped in rich beef stock, tarragon mustard, cider vinegar, and sunflower oil until luscious and schlotzig.
Peel the potatoes while still hot from boiling (hold them with a cloth or fork). Slice immediately into 4mm rounds into a large wide ceramic bowl; warm potatoes absorb the dressing deeply.
In a jug, whisk the hot beef stock with the cider vinegar, tarragon mustard, sugar, salt, and finely minced red onion until completely emulsified.
Pour the hot dressing over the sliced potatoes. Carefully toss with a silicone spatula or swirl the bowl, letting the starch bind with the stock. Allow to steep for 20 minutes to absorb.
Fold in the sunflower oil last to give the salad its famous creamy, silky sheen ('schlotzig'). Scatter with fresh snipped chives and serve lukewarm with authentic Wiener Schnitzel.