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Heat the oil in a large saucepan. Add the onions, beetroot and a pinch of salt and cook for 10 mins, then turn up the heat and add the chilli flakes and 1 tbsp each of the cumin and coriander seeds. Cook until aromatic, then add the vinegar and give everything a good stir. Pour in the stock and bring to the boil, then cover and simmer for 45 mins-1 hr until a knife can be easily inserted into a beetroot wedge; uncover the pan halfway through cooking to reduce the soup.
Meanwhile, make the dukkah. Put the hazelnuts, sesame seeds and the remaining cumin and coriander into a dry frying pan and gently toast until the hazelnuts are golden. Add a pinch of salt and crush with a pestle and mortar (alternatively, use a knife to roughly chop the nuts).
Blitz the soup with a hand blender and season to taste. Divide between four bowls, then top with a swirl of the yogurt and a sprinkling of hazelnut dukkah.