Add the leeks, potatoes and onion
and stir well to coat with the butter.
Cover tightly with a piece of
greaseproof paper and cook over a very low heat, stirring
frequently, for about 15 minutes, or until softening, but not
colored.
Add the stock or water and milk
and season to taste with salt and pepper.
Bring to the boil, reduce the heat
and simmer gently for about 20 minutes until the vegetables are
tender.
Transfer the soup to a food
processor and process until smooth.
Return the soup to the saucepan,
adjust the seasoning if necessary and heat through.
Pour into individual bowls,
garnish with chives and serve immediately.