Makes 4 servings.
2/3 cup chopped fresh basil
4 tablespoons plain nonfat yogurt
Heat oil in heavy medium saucepan over medium heat. Add onion; sauté until tender, about 5 minutes. Add cumin; stir until aromatic, about 30 seconds. Mix in zucchini. Add broth; bring soup to boil. Reduce heat; simmer until zucchini is very tender, about 30 minutes. Cool slightly.
Mix 1/3 cup chopped basil into soup. Working in batches, puree soup in blender until smooth. Transfer to bowl. Season with salt and pepper. Cover; chill until cold, about3 hours. (Can be made 1 day ahead. Keep refrigerated.)
Ladle soup into bowls. Top each with 1 tablespoon yogurt and sliced basil.