Swiss Chard Soup with Lemon

I still have lots of greens from last summer's garden in the freezer, so I went looking through my cookbooks to find a soup recipe that would use some of them. Deborah Madison's great cookbook Vegetarian Cooking for Everyone came through for me, with a recipe that looked very good, using swiss chard. I like to freeze lots of chard so I can make this pizza throughout the winter. This soup will become another seasonal favorite; it's easy to make and is tangy with the flavor of lemon juice.

2 Tbs butter
1 onion, chopped
3 red potatoes, peeled and thinly sliced
1 bunch chard, stems removed, about 10 cups
2 cups sorrel leaves, stems removed, or juice of 1 large lemon (I used the lemon juice since I have no sorrel during winter.)
salt and freshly ground pepper
1/3 cup creme fraiche or sour cream
1/2 cup cooked rice or small toasted croutons

1. Melt butter with onions and potatoes in a soup pot over medium high heat. Cook, stirring occasionally, under the vegetables begin to brown, about 8 minutes.
2. Add 1/2 cup water and scrape bottom of pot to release juices.
3. Add greens and 1 1/2 teas salt; cook until they wilt.
4. Add 6 1/2 cups water (I added a little less since I like a thick soup), bring to a boil and simmer, partially covered, for 12 to 15 minutes.
5. Let the soup cool a bit, then puree it and return to the pot.
6. Taste for salt and season with pepper. Add lemon juice if you didn't use sorrel.
7. Mix the sour cream or creme fraiche with a little bit of soup to smooth it, then add to the soup. 
8. Serve with rice or croutons in each bowl. (I like to make croutons with my home made sourdough bread, frying it in olive oil.)