Melt the butter in a large saucepan, add the leeks, potatoes and onion. Stir well so they are covered in the butter. Cook over a very low heat for about 15 minutes, until the vegetables are soft, stirring to stop them from becoming brown.
Add the chicken stock, milk and bouquet garni and a dash of salt and pepper. Bring to the boil, then turn down the heat and simmer gently for about 20 minutes until all the vegetables are tender.
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