Saute leeks in hot oil in a Dutch oven until tender. Add zucchini and broth. Bring to a boil; cover, reduce heat, and simmer 8 to 10 minutes or until zucchini is tender. Remove from heat; cool slightly.
Process mixture in batches in a blender until smooth, stopping to scrape down sides.
Stir in half-and-half and next 3 ingredients. Chill at least 3 hours. Garnish, if desired.