Melt the butter in a pan, add the leeks and onion and fry for 5 minutes, without browning.
Add the potato and cook for 2 minutes.
Meanwhile, remove the tough stalks from the watercress and roughly chop the leaves.
Add to the pan with the stock, and salt and pepper to taste.
Bring to the boil, then cover and simmer for 25 to 30 minutes.
Sieve or work in an electric blender until smooth.
Pour into a bowl and stir in the milk.
Chill for several hours before serving.
Serve garnished with croutons.