How to Make It
Bring a large saucepan of salted water to a boil. Add the potatoes and cook until barely tender, about 2 minutes. Add the leeks and cook until the potatoes are tender, about 2 to 3 minutes longer. Drain the potatoes and leeks and cool under running water. Drain again and pat thoroughly dry with paper towels.
In a medium bowl, whisk the crème fraîche with the sherry vinegar and chives and season with salt and white pepper. Fold in the potatoes and leeks and serve.