Melt the butter in a large shallow frying pan, add the carrot and season with salt and pepper. Add the nutmeg or coriander. Cover the pan with a sheet of greaseproof paper and a lid. It is important to cover to prevent loss of steam made by the carrots as they cook, or they will dry and turn brown.
Cook over low heat for 15 minutes, or until very soft and tender enough to be mashed with a fork, then remove the paper and lid. Cook, uncovered, over high heat to reduce any excess moisture, then cool slightly. Purée in a food processor until smooth. Return to the pan, adjust the seasoning and reheat to serve. Serve the purée as oval quenelle shapes by pushing a rounded dessertspoon of purée off the spoon using another spoon, both held horizontally, or simply serve it in a neat mound.