"Creamy fingerling potatoes seasoned with dill and red onions have a tangy sour cream-based dressing. [Photographs: J. Kenji Lopez-Alt] The key to the deeply-seasoned potatoes in this tangy and creamy salad is to cook them in water seasoned with salt and vinegar. A dressing flavored with olive oil, sour cream, and dill binds it together. Why this recipe works: * Adding salt and vinegar to the potato cooking water not only seasons the potatoes deeply, but the vinegar also prevents them from falling apart as they cook. * Seasoning the potatoes with more vinegar while they're still hot is key to deep flavor penetration and a light texture on the palate...."