To prepare the soup, heat a large saucepan over medium-high heat. Coat pan with cooking spray. Add leek and celery; sauté 4 minutes. Add zucchini; sauté 5 minutes. Add broth; bring to a boil. Partially cover, reduce heat, and simmer 20 minutes or until zucchini is very tender. Remove from heat, and cool slightly. Place half of zucchini mixture in a blender; process until smooth. Pour into a medium bowl. Repeat procedure with remaining zucchini mixture. Stir in buttermilk, salt, and pepper.
To prepare pesto, place watercress and remaining ingredients in blender; process until smooth (add more water if necessary). Drizzle pesto over the soup.