2 3/4 pounds baby Yukon Gold or baby red potatoes (about 2 inches each),
1/2 cup mayonnaise
2 tablespoons distilled white vinegar
2 teaspoons Dijon mustard
Freshly ground pepper
2 scallions, thinly sliced
2 tablespoons chopped flat-leaf parsley
1 celery rib, cut into 1/4-inch dice
In a large saucepan, cover the potatoes with cold water and season the water with salt. Bring to a boil over high heat and cook the potatoes until tender, about 15 minutes. Drain and let stand until cool enough to handle, about 10 minutes.
Meanwhile, in a large bowl, whisk the mayonnaise with the vinegar and mustard and season with salt and pepper. Stir in the scallions, parsley and celery. Halve the potatoes crosswise and fold them into the dressing. Serve warm or at room temperature.
The potato salad can be refrigerated up to 1 day. Bring to room temperature before serving.