Coarsely chop watercress leaves and stems separately. 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 Puree 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 and transfer to bowl. Serve hot or chill until cold, about 4 hours.

starter during the colder months. Recipe uses both stems and leaves.