Melt the butter in a pan over a medium to low heat and add the leeks, potatoes and onion stirring well. Season with salt and pepper. Allow the vegetables to sweat for 15 minutes over a very low heat.

Add the stock and milk and bring to a simmering point. Cover and let the soup simmer gently for 20 minutes, or until the vegetables are soft, don't allow the milk to boil.

Allow to cool slightly then blend into a smooth creamy texture. Return to the heat, check seasoning, add more salt and pepper if needed. Add a tablespoon or two of creme fraiche depending on how creamy you want it.