I found this recipe here (http://www.independent.co.uk/life-style/food-and-drink/recipes/preformleeks-in-white-sauceb-preform-663639.html) and I've been using it for quite some time. It is the best one I've found. Enjoy!
500ml milk
2 cloves
1 onion, peeled and chopped
1 bay leaf
salt and pepper
75g butter
50g flour
150ml single cream
freshly grated nutmeg
2 cloves
1 onion, peeled and chopped
1 bay leaf
salt and pepper
75g butter
50g flour
150ml single cream
freshly grated nutmeg
8 large leeks, trimmed of almost all their green parts, sliced into 5cm lengths and thoroughly washed.
Heat together the milk, cloves, onion, bay and a little salt. Simmer for a few minutes, cover and allow the flavours to mingle for 10 minutes. In another pan, melt the butter and stir in the flour. Make a roux and gently cook the butter and flour together for a minute or so, but don't allow it to colour. Strain the milk into the roux and vigorously whisk together until smooth. On the lowest possible heat (preferably with a heat-diffuser pad), set the sauce to cook.
You might think that the sauce is very thick to begin with but, as it simmers, the texture will become silky and unctuous. Remember, cream is added later, too. Do not cover the sauce as it cooks, but stir from time to time with a wooden spoon and continue in this fashion for 20 minutes or so. Finally, add the cream, nutmeg and pepper, mix in thoroughly, check for salt and cook for a further 5 minutes. Cover and keep warm.
Switch the kettle on and put the leeks into a pan. When the water has boiled, pour it over the leeks and add a little salt. Bring back to the boil and cook them for between 5 and 10 minutes, depending upon how thick your leeks are; test with a small, sharp knife for tenderness. You don't want crunchy leeks; nor do you want sloppy ones. Drain, lay in a preheated, shallow, oven-proof dish and pour over the white sauce.