Cook the potatoes in a large saucepan of salted boiling water for 10–15 minutes or until tender.
Meanwhile, combine the fennel, celery and onion in a large, shallow bowl. To make the dressing, mix all the ingredients together in a small bowl and season to taste with salt and pepper.
Drain the potatoes, rinse under cold running water, then drain again. Add the potatoes to the salad. Add the dressing and toss until well coated. Garnish with celery leaves or dill sprigs, if liked, before serving.