Heat the remaining oil in a 12-quart saucepot over medium-high heat. Add the leeks and garlic and cook until the leeks are tender. Stir the broth, cilantro, curry powder, ginger root, lemon zest, cumin and cauliflower in the saucepot and heat to a boil. Reduce the heat to low. Cover and cook for 15 minutes or until the cauliflower is tender.
Pour one third of the cauliflower mixture into a blender or food processor. Cover and blend until the mixture is smooth. Repeat with the remaining cauliflower mixture. Pour the soup through a sieve into a large bowl. Season to taste.