In a large thick based saucepan gently melt the butter.
Add the chopped up leeks, onion and potato and stir them around to 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 mins. Do not let them burn.
After that add the stock and the milt and bring the soup slowly to simmering point. Cover and let the soup simmer very gently for another 20 mins or so until the vegetables are soft.
Once cooked allow it to cool then pop it into a blender.
Serve with chopped parsley or chives and a swirl of cream.
Omitting the milk in the cooking gives a less creamy soup.
Tinker with the basic soup by adjusting the proportions of the vegetables, add chopped carrots etc.