Wash the beetroot, put in a pan (unpeeled), cover with water and bring to the boil. Cook until tender (approx. 40 mins), topping up the water, if necessary. The beetroot is ready when a sharp knife goes through easily.
Drain and leave to cool. Peel the beetroot and grate using the coarse side of a grater.
Transfer to a bowl, add the yoghurt, garlic, oil, mint, vinegar, feta, nigella seeds, salt and pepper and mix well.
Top with a sprinkling of nigella seeds and a drizzle of oil. Serve with Flatbread.