Heat the oil in a large pan, add the onion, then fry for 5 mins until softened. Stir in the ground coriander and potato, then cook for 1 min. Add the carrots and stock, bring to the boil, then reduce the heat. Cover and cook for 20 mins until the carrots are tender. Tip into food processor with the coriander then blitz until smooth (you may need to do this in two batches). Return to pan, taste, add salt if necessary, then reheat to serve.