How to Make It
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.