The secret of this honest soup lies in the initial ‘sweating’ of the leeks and onions - this involves very slow cooking with a little butter in a covered pot, to release the vegetable juices.
Trim the leeks just above the root and wash thoroughly, checking between the layers for any trapped grit. Cut in half lengthwise and wash again, if necessary, then slice thinly. Slowly melt the butter in a heavy-based 4-litre saucepan, add the leek and onion and coat with the butter. Cover the pan and sweat really slowly until they are very soft. On no account should the