Wash the beetroot carefully without piercing the skin. Put in a saucepan of cold water and bring to the boil. Cook for 30-60 minutes depending on the size of the beetroot. They are ready when you can slip a sharp knife easily into the centre.
Drain and rub away the skin with your fingers under a running cold tap. Cut into 1 cm rounds, arrange on a plate and dress with the lemon juice, olive oil, parsley, salt and pepper.
Mix the garlic with the yoghurt and check the seasoning. Pour over the beetroot, drizzle with a little olive oil and sprinkle on the nigella seeds.