Melt butter in large pot over medium heat. Add celery, onion, fennel, garlic and ginger. Cover pot. Cook until vegetables are almost tender, stirring occasionally, about 20 minutes. Add broth and wine. Bring to boil. Cover and simmer until vegetables are tender, about 10 minutes. Remove from heat and mix in watercress. Let stand 5 minutes. Working in batches, puree soup in blender until smooth. Transfer to large bowl. Season to taste with salt and pepper. Cover and refrigerate until cold, at least 4 hours. (Can be prepared 1 day ahead. Cover and keep refrigerated.)
Ladle soup into bowls. Top with dollop of yogurt, if desired.
