Gratin de Poireaux au Roquefort

• 1 kg/2¼ lb small tender leeks
• 25 g/1

Cook the leeks in boiling salted water until tender. Lift them out of the water (reserve the water) and leave to drain in a warm place so that they do not get cold.

Melt the butter in a saucepan, stir in the flour and cook for 2 minutes to make a roux. Add the milk and 200 ml/