Step 1Wash the sorrel, discarding bruised or blemished leaves. Trim tough stems. Dry with paper towels or a salad spinner.
Step 2Stack the leaves and roll them lengthwise, then slice them crosswise into half-inch-thick strips.
Step 3Heat the butter in a large saucepot. Add the leeks and cook until tender, about 3 minutes. Add the garlic and saute 1 minute.
Step 4Add the sorrel and saute about 1 minute, until the sorrel wilts and turns olive green. Reduce the heat to medium low and cook 2 minutes.
Step 5Add the chicken broth (4 cups if using the whipping cream, 4 1/2 cups if omitting the cream) and bring the mixture to a boil. Reduce the heat and simmer 30 minutes.
Step 6Remove from heat. Stir in the whipping cream, if using. Season with sugar, salt and pepper.
Step 7Puree the soup in a blender, in batches if necessary, until smooth and velvety. To serve, ladle into bowls and sprinkle with chopped chervil or chives.