Cream of Sorrel Soup

It's been chilly and damp for a few days, so my thoughts of food go to soup. Since I have an abundance of the lemony perennial herb Sorrel in the garden, I made a cream of sorrel soup. It is rich with butter, broth and cream, so the sharp taste of the sorrel is muted and just piquant enough. The recipe comes from an old soup cookbook of mine, A Celebration of Soups, out of print but still available used. I garnish the soup with croutons made from my sourdough bread. I've been enjoying it for lunch for the past few days; it's comforting and elegant at the same time. I've been having it hot, but it's also good chilled on a midsummer's day.

3 Tbs butter
1/2 lb sorrel leaves, stemmed, rinsed and chopped
4 cups chicken stock. (I used some stock from cooking chick peas, very delicious. I've also made this with water, and it's still good but not as rich.)
3 egg yolks
1 cup heavy cream
salt
white pepper (the cookbook author, Robert Ackart, likes to use white pepper so as not to spoil the look of the soup with dark specks)
croutons for garnish

1. Heat the butter in a large saucepan and cook the sorrel leaves for 10 minutes or so, until they cook down to a gray green soft mass.
2. Whirl the sorrel in a food processor until smooth; return to saucepan and add the stock. Bring it to a boil, reduce the heat and simmer, covered, for 5 minutes.
3. In a mixing bowl, beat the egg yolks with the cream using a rotary mixer. Add one cup of the simmering broth, mixing. 
4. Beat the egg/cream mixture into the stock, not allowing it to boil.
5. Season to taste with salt and pepper. Serve garnished with croutons.