Finely shred the red and white cabbage using a sharp knife or a mandoline. Place in a large mixing bowl and add the chopped chives. Mix well.
Combine the yoghurt, wholegrain mustard and cider vinegar in a separate bowl. Mix well and season with salt and pepper to taste.
Pour the dressing over the shredded vegetables, toss well and serve immediately. Do not dress the coleslaw in advance or the cabbage will release water and go soft.
