Delia, "Complete Cookery Course", pg 97
Split the leeks in half lengthways and slice finely. Wash well.
Melt the butter & oil, add the leeks, potato and onion and stir to coat. Season and let the vegetables sweat over a very low heat for about 15 mins.
Add the stock and milk, bring to simmering point and simmer gently for a further 20 mins or until the vegetables are soft.
Liquidise, and taste to check seasoning.
Serve with chopped chives.