Preparation Coarsely chop watercress leaves and stems seperately. Melt butter in heavy large saucepan over medium-low heat. Mix in onion, parsley sprigs, thyme sprigs, garlic, bay leaf, and watercress stems. Cover; cook until onion is tender, stirring occasionally; about 12 minutes. Add broth and potatoes; bring to boil. Reduce heat cover and simmer until potatoes are very tender, about 25 minutes. Discard parsley, thyme, and bay leaf. Purée soup in processor until smooth. Return soup to same saucepan. Add 1 cup milk and watercress leaves; bring to simmer, adding more milk if soup is too thick. Season with salt and pepper. Transfer to bowl. Chill until cold, about 4 hours.
Preparation Coarsely chop watercress leaves and stems seperately. Melt butter in heavy large saucepan over medium-low heat. Mix in onion, parsley sprigs, thyme sprigs, garlic, bay leaf, and watercress stems. Cover; cook until onion is tender, stirring occasionally; about 12 minutes. Add broth and potatoes; bring to boil. Reduce heat cover and simmer until potatoes are very tender, about 25 minutes. Discard parsley, thyme, and bay leaf. Purée soup in processor until smooth. Return soup to same saucepan. Add 1 cup milk and watercress leaves; bring to simmer, adding more milk if soup is too thick. Season with salt and pepper. Transfer to bowl. Chill until cold, about 4 hours.