Put the beetroot, celery, mushrooms, onions, pepper and potatoes into a large saucepan and stir in the oil. Cook over a high heat, stirring, until the vegetables start to sizzle. Cover the pan, reduce the heat to low and simmer for 10 minutes without lifting the lid.
Stir in the coriander and cumin and cook for a further 1-2 minutes. Add the stock and thyme, bring to the boil, then lower the heat and simmer for 20 minutes, stirring occasionally.
Season with salt and pepper to taste. Pour the soup into bowls and garnish each serving with curd and a sprinkling of chives.
Variation: to make a smooth soup, purée all the ingredients at the end of Step 2 in a food processor or by using a hand-held mixer.