Preheat the slow cooker if necessary; see the manufacturer's instructions. Heat the oil in a large frying pan, add the onion and fry over a medium heat, stirring, until softened. Stir in the carrots, cumin seeds and turmeric and fry for 2–3 minutes to release the cumin flavour and colour the onions.
Stir in the rice, then add the stock and a little salt and pepper and bring to the boil. Pour into the slow cooker pot, cover with the lid and cook on low for 7–8 hours or until the carrots are tender.
Purée the soup while still in the slow cooker pot with a stick blender. Alternatively, transfer to a liquidizer and purée, in batches if necessary, until smooth, then return to the slow cooker pot and reheat on high for 15 minutes.
Taste and adjust the seasoning, if needed, then ladle the soup into bowls. Top with spoonfuls of yogurt and a little mango chutney and serve with poppadums.