Trim off the tops and roots of the leeks and peel away their tough outer layer. Cut the leeks in half and rinse well under cold water. Then chop into little pieces (the soup will be blended so just roughly chop them up!)
Gently melt the butter in a heavy-bottomed pan.
Add the leeks and onion, stir until soft (about 5 minutes). Add the potatoes after a couple of minutes and saute them.
When they are coated in butter, add the stock, milk and sugar (optional). Let the soup simmer for 20 minutes or until cooked.
Blend the soup in a food processor or blender after it as cooled a little.
Reheat the soup on the stove and serve (if it is too thick, add more milk).