Potato Leek Spinach soup

4 large potatoes (I used large bakers, all I had in the house)
2 large leeks, white and tender green parts only
3/4 pound mushrooms (I used plain white button mushrooms)
3/4 pound fresh spinach
salt
butter buds (optional)
skim milk (optional)
Peel and coarsely chop potatoes. Trim leeks, slice in half longwise and
separate under running water to remove all sand and dirt, then coarsely
chop. Clean and quarter mushrooms. Place all in large pot and just
cover with water. Add salt, about 2 teaspoons, and bring to boil.
Simmer until potatoes are tender, about 20 minutes. Ladle 1/2 of cooked
potato mixture into food processor and add half of the spinach. Process
until smooth and thick. Repeat with rest of potatoes and spinach. Add
about 1 Tablespoon butter buds for flavor and thin with skim milk if needed.
Serve hot. Makes about 6-8 servings (although 2 of us ate at least half
of it at one sitting, with just fresh crusty french bread along side!)

Notes on variations from Mollie's original version: I thought I used too
much water at first and removed some before I processed the soup. It was
then thicker than I wanted, so I thinned it with the milk. The milk and
butter buds were not in Mollie's original version. She used watercress,
but suggested that spinach, or any other green, would also be good.