Kosher salt and freshly ground black pepper, to taste
2 tablespoons whole-grain mustard
¼ cup cider vinegar
Pinch of cayenne pepper
3 tablespoons finely chopped chives
Directions
In a large skillet over medium-low heat, warm 1 tablespoon of the olive oil. Add the onion and cook, stirring often, until translucent and just tender, 8 to 10 minutes. You don't want the onion to brown. Transfer the onion to a bowl and let cool.
In a large saucepan, cover the potatoes with water by 1 inch and season generously with salt. Bring to a boil, reduce the heat to a simmer and cook until the potatoes are tender, 25 to 30 minutes. Drain the potatoes and transfer them to a cutting board.
While the potatoes are cooling, in a large mixing bowl, whisk together the mustard, vinegar and cayenne. Slowly whisk in the remaining ¼ cup of olive oil. Season with salt and pepper.
When the potatoes are cool enough to handle, cut them into ½-inch dice.
Toss the warm potatoes and reserved onions with the vinaigrette. Add the chives, season with salt and pepper, and toss again. Serve immediately or refrigerate for up to 1 day.