Cook the leeks in a pan of boiling salted water for 7 to 10 minutes until just tender. Drain and plunge them immediately into icy cold water. After 15 minutes, drain the leeks again and dry well with several layers of kitchen paper. Put the leeks in a food processor or blender and whiz until roughly chopped. Remove half and set aside, then whiz the remainder until smooth.

Heat the butter in a frying pan, add all the leeks and stir over a high heat. Add the creme fraiche and season; stir until hot and bubbling. Serve with a sprinkling of grated nutmeg.