Ever since settlers in the first Southern colonies churned butter derived from a sour milk called clabber and drank the leftover nutritious liquid, buttermilk has had a special status in the South that few outsiders appreciate. Gone are the days, however, when you can find natural buttermilk like that I drank as a child. Today it is replaced by the very respectable commercial product made from low-fat milk transformed with special bacteria. For centuries, buttermilk has been a vital ingredient in the making of certain biscuits, cornbreads, hush puppies, pies, puddings, spoonbreads, marinades, and salad dressings, but it’s also the major ingredient in any number of tangy soups, such as this one, which is just as good chilled as hot. Fresh parsley can be substituted for the watercress, and if you want to produce a really modern soup, use arugula leaves.
Tear off about
In a saucepan, melt the butter over moderate heat, add the onions and garlic, and stir till soft, about 2 minutes. Add the potatoes, broth, and salt and pepper and bring to a boil. Add the remaining watercress, reduce the heat slightly, cover, and simmer till the potatoes are tender, about 15 minutes.
Meanwhile, bring about
Pour the potato-and-watercress mixture into a blender or food processor, blend thoroughly, and return to the saucepan. Add the buttermilk, cayenne pepper, and blanched watercress leaves, heat thoroughly, and serve hot or warm in soup plates.
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