Here is a wonderful zucchini soup that gives welcome relief in the hot and humid Louisiana summers. It's a velvety-smooth soup with no cream, so it's easy on the calories, but it tastes like an indulgance. ~- ~- In a medium saucepan, melt the butter and saute the onion until tender, about 2 minutes. Add the broth, zucchini, watercress, potato, parsley and Tabasco sauce; stir well. Bring to a boil, reduce heat, cover and simmer for 20 minutes, until the potato is soft. Cool. Puree the soup in two batches in a blender or food processor. Chill. Serve with yogurt or sour cream.