Put the potatoes in a big pot and cover them with water. Add a couple of pinches of salt, then bring the water to a boil.
Cook for about 15 minutes, until the potatoes are just tender (pierce one with a fork to check). Drain the potatoes and let them cool.
Meanwhile, finely chop the celery and onion. Mix the dressing.
When the potatoes are cool, cut them in half (if big), then put them in a serving dish. Add the onion and celery, then pour over the dressing and gently combine. Taste and adjust the seasoning.