Wash the potatoes thoroughly and boil in plenty of water for 15-20 minutes. In the meantime, wash the celery and put some greenery aside for garnishing. Cut the celery into thin slices. Wash the tomatoes, dab dry and cut into quarters. Remove the seeds and cut the quarters into fine cubes.
Wash the chives, dab dry and cut into fine rolls. Mix vinegar, 2 tablespoons water, mustard, salt and pepper. Finally, fold in the oil. Mix celery, tomatoes and chives with the vinaigrette. Drain the potatoes, rinse with cold water and peel. Cut lukewarm potatoes in half lengthwise and mix with the vinaigrette. Leave to stand for 15-20 minutes. Pierce the egg and boil in plenty of water for 7-8 minutes. Rinse under cold water, peel and halve.
Cut lukewarm potatoes in half lengthwise and mix with the vinaigrette. Leave to stand for 15-20 minutes. Pierce the egg and boil in plenty of water for 7-8 minutes. Rinse under cold water, peel and halve. Season potato salad with salt and pepper again and arrange the egg on top. Serve garnished with celery green