Heat the oil in a heavy-bottomed saucepan and sauté the onion until soft, then add the garlic and ginger, and stir-fry over low heat.
Add the coriander, cumin, cabbage and beetroot, pour the stock over, and then add the tomatoes and brown sugar. Bring to the boil, then simmer until the beetroot is soft. Blend the soup until smooth and serve with a swirl of Greek yoghurt and chopped, roasted hazelnuts.