Creamy spinach soup

1 large onion, coarsely chopped
 6 cups water
 3 potatoes, peeled and chopped
 3 zucchini, thickly sliced
 2 cups tightly packed washed and dried fresh spinach leaves
 Several twists of freshly ground pepper
 1/4 package (3.5 ounces) enoki mushrooms (optional)
 Place the onion in a large pot with 1/2 cup of the water.  Cook and stir
 until the onion softens slightly, about 3 minutes.  Add the remaining water
 and the potatoes, zucchini, and soy sause.  Bring to a boil, reduce the
 heat, cover, and simmer for 35 minutes.  Add the spinach and pepper.  Cook
 for another 2 minutes.  Remove from the heat.  Puree the soup in batches in
 a blender and return to the pan.   Add the mushrooms, if desired.  Heat
 gently for 5 minutes.  Serve hot.
 This is how the recipe appears in the book.  I have noted some additions
 from a posting here on the FF list to use 2 bunches of fresh spinach, and to
 add cayenne pepper and garlic powder to taste.