Perfect for the subscription bag in winter!
Begin by trimming the leeks, discarding the tough outer layer.
Now split them in half lengthways and slice them quite finely, then wash them thoroughly in two or three changes of water. Drain well. In a large, thick-based saucepan, gently melt the butter, then add the leeks, onions and potatoes, stirring them all round with a wooden spoon so they get a nice coating of butter. Season with salt and pepper, then cover and let the vegetables sweat over a very low heat for about 15 minutes.
After that, add the stock and milk, bring to simmering point, cover and let the soup simmer very gently for a further 20 minutes or until the vegetables are soft – if you have the heat too high the milk in it may cause it to boil over.
Now you can put the whole lot into a blender – leave it to cool a little first – and blend to a purée. If you have to blend the soup in batches, make sure you have a bowl to hand to put the first batch into. Now return the soup to the saucepan and reheat gently, tasting to check the seasoning. Add a swirl of cream or crème fraîche before serving and sprinkle with freshly snipped chives or parsley
From: Delia Smith.