Directions
Preparation:20min  ›  Cook:45min  ›  Extra time:10min  ›  Ready in:1hour15min 

1. Use a large knife to cut off the tops of the leeks, then shave off any tough parts and remove any coarse outer leaves. Trim off the roots and cut each leek in half lengthways. Rinse thoroughly under running water and slice thinly.
2. Melt the butter in a large saucepan, then add the leeks. Toss them in the butter for 3–5 minutes, then lay the potatoes on top. Don't stir the potatoes through the leeks because they can stick to the bottom of the pan. Cover the pan with a lid and cook on very low heat for about 20 minutes.
3. Stir in the salt, then add the water and hot milk. Bring the soup just to the boiling point, stirring often, then immediately turn the heat to low. Partly cover with a lid and cook very gently for 20 minutes more. Purée in batches in a food processor or blender and return the soup to the cleaned pan.
4. To give the soup a smooth finish, prepare the liaison. Blend the egg yolks and cream in a small bowl and slowly add a ladleful of the hot soup, whisking constantly. Pour this, stirring, into the soup and set the pan on medium heat. Keep stirring until the soup thickens slightly (do not allow it to boil or it will curdle).
5. If you are not using the liaison, reheat the soup until it is piping hot. Ladle into bowls and garnish with a little chopped parsley or a few snipped chives.
6. For a change, sizzle a few rashers of bacon, rind removed and finely chopped, and sprinkle on top of the soup before serving.