• Trim, wash and slice the leeks in half lengthways. Put the leeks in a large saucepan, add a little salt, then pour in enough water to just cover the vegetables.
• Bring to the boil, then simmer for 10 minutes. Drain the leeks well and reserve 150 ml (1/4 pint) of the cooking water for the sauce. Place in a serving dish and keep hot.
• To make the sauce, melt the butter or margarine over a gentle heat, then stir in the flour and cook for 2 minutes.
• Gradually add the milk and leek water, stirring the mixture all the time. Bring the sauce to the boil and simmer gently for a few minutes.
• Remove from the heat, stir in the Cheddar, seasoning and mustard. Mix until smooth and pour the sauce over the leeks. Serve at once.

Cook's Tip

• Substitute dry white wine for cooking water and use half the quantity of cheese to make a wine sauce. Or use all milk to make a creamy sauce.