Place the red onion in a small bowl and cover with the vinegar. Set aside while the potatoes cook.
Place the potatoes into a pan of cold salted water. Bring to the boil then lower the heat and simmer for 15 minutes until tender. Drain well and leave to cool.
In a bowl, combine the yoghurt, mayonnaise, mustard and mix well.
Pour 30ml of the vinegar into the yoghurt mixture and mix well.
Discard the remaining vinegar and add the red onion to the yoghurt mixture along with the gherkins.
Once cooled, add the potatoes to the yoghurt mixture and stir to coat.
Season to taste with salt and pepper and sprinkle over the chives.