Peel the beetroot and chop into bite-size pieces. Cook in a pot of salted boiling water until a knife goes through easily. Meanwhile, toast the hazelnuts with a pinch of salt in a pan over medium heat, stirring or shaking so they don't burn. Pound them coarsely in a mortar and pestle. Whisk the yoghurt, olive oil, balsamic vinegar and salt in a small bowl. Finely chop the garlic and stir it in. Thin with 2 tablespoons of water and chill. Drain the beetroot, rinse with cold water, drain again and leave to cool in a bowl. Peel the oranges with a sharp knife, removing all the pith, and slice into circles, discarding any seeds. Pour the dressing over the cooled beetroot and toss by hand until coated. Add the hazelnuts and toss briefly. Arrange the beetroot on a plate, top with the orange slices and season to taste.