Heat the oil in a wide heavy pot. Add the leeks and sauté until translucent. Add the celery, zucchini, turmeric, water, wine and salt, and bring to a boil. Reduce the heat to medium and cook, covered, 30 minutes. Stir in the watercress and cook only a few seconds, until wilted. Turn off the flame. The remaining ingredients and cream the soup with an immersion blender. Adjust the texture and seasonings. Chill the soup.

Tip: The immersion blender is a wonderfully nifty tool, inexpensive and portable (it will fit in a drawer), that allows you to blend your soup directly and in one shot right in your pot. No transferring, no mess. Just make sure there are no bones in the soup, or you will break your blade.