In a heavy kettle cook the fennel bulb and the onion in the oil, covered, over moderately low heat, stirring occasionally, for 10 minutes, add the broth, the water, and the fennel seeds, and simmer the mixture, covered, for 10 to 15 minutes, or until the fennel bulb is tender. Stir in the spinach and simmer the soup for 1 minute, or until the spinach is wilted. In a blender or food processor puree the soup in batches. The soup may be served hot or chilled. Makes about 8 cups, serving 6 to 8.