I must have forgot the spinach at the store so I substituted some sliced yellow squash, some snap peas (added toward the end of the mushroom saute). We used chicken stock. Added some thinly sliced basil just before serving. We also had 2 eggs on top of each serving - souper good <get it?> Will make again! So easy and like Suzanne said, you can easily customize it according to what you've got on hand.
This rating is for the soup with out the egg (we had already had eggs that morning) and with a few changes. I made the soup as written with the following changes: I used canned tomatoes (not the season for fresh) and added some carrots and green beans to use them up. I also added a few dashes of pepper sauce. The addition of the cider vinegar was perfect. This was great cool weather fare and could be customized in many ways. Very good!