While still warm, these potatoes are tossed with a tangy mustard and vinegar mixture; this way, they soak up the flavors as they cool.
Place potatoes in a medium saucepan, and cover with cold salted water. Bring to a boil, reduce to a simmer and cook, until just tender, 12 to 14 minutes. Drain; set aside to cool slightly.
Meanwhile, in a medium bowl, combine vinegar, Dijon, and whole-grain mustard; season with salt and pepper.
When potatoes are cool enough to handle, quarter them, and toss with mustard-vinegar mixture. Refrigerate until completely cold, about 2 hours. Add oil, celery, and celery leaves to potatoes; toss and serve.