Melt the butter in a medium Dutch oven or pot over medium heat. Add the leek and celery; season with salt and pepper. Cook, stirring occasionally, until the leeks are wilted, about 5 minutes. Add the thyme and garlic, then sprinkle in the flour. Cook, stirring, until the vegetables are coated with flour, 1 to 2 minutes.
Add the chicken broth, artichokes, bay leaf and lemon zest to the pot and bring to a simmer. Cook until the vegetables are very tender, 10 to 12 minutes. Stir in 1/2 cup heavy cream and the spinach; cook until the spinach is wilted, about 4 minutes. Remove the bay leaf and lemon zest.
Working in small batches, transfer the soup to a blender and puree with the parsley and 1/4 cup creme fraiche until smooth. Return to the pot, season with salt and reheat.
Combine the remaining 2 tablespoons creme fraiche and 2 teaspoons heavy cream. Ladle the soup into small glasses; top with the thinned creme fraiche and chives.
Photograph by Ryan Liebe
Courtesy of Food Network Magazine