If you can't find sorrel for this soup—adapted from an Alice Waters recipe published in Chez Panisse Vegetables (HarperCollins, 1996)—use more watercress and a little fresh lemon juice instead.
4 tbsp. butter
1 large yellow onion, peeled and
1 medium carrot, peeled and
1 sprig fresh thyme
3⁄4 lb. white boiling potatoes, peeled
and coarsely chopped
1 1⁄2 cups Chicken Stock
Salt and freshly ground black pepper
8 cups sorrel leaves
2 1⁄2 cups watercress leaves, plus
additional for garnish
1⁄3 cup half-and-half
Sour cream (optional)
1. Melt butter in a large stainless-steel saucepan over medium-low heat. Add onions, carrots, and thyme and cook, stirring occasionally, until carrots are tender and onions soft, about 20 minutes. Add potatoes, stock, and 3 1⁄2 cups water. Season with salt and pepper and bring to a simmer over medium-high heat. Reduce heat to medium-low and cook, partially covered, until potatoes are tender, about 20 minutes more.
2. Increase heat to medium-high, add sorrel and watercress, bring just to a simmer, then remove from heat. Allow soup to steep for 10 minutes, then purée in batches in a blender. Strain through a fine sieve back into saucepan, stir in half-and-half, and season to taste with salt and pepper. Warm gently over medium-low heat. Serve garnished with watercress leaves and sour cream if desired.