Over the centuries when potatoes were one of the only vegetables available through the lean winter months, this refreshing soup, from Germany's Black Forest region, must have tasted like a miracle. Grated horseradish (another long-storing root) gives it an invigorating zing, and though the combination of potatoes and cream might sound heavy, one of the charms of this soup is that it's actually quite light. These days, an immersion blender is used to whip it into a froth, and the starch from the potatoes helps to set the air bubbles and maintain that ethereal consistency. Soups like this one, served with a garnish of garlicky Fleischwurst sausage, are classic German pub fare, and in the Palatinate region, in the western part of the country, they are traditionally eaten for lunch along with a yeasty cake called Quetschekuche, made with small, blue, sweet-sour plums. Then again, as airy and elegant as this soup is, it also makes a wonderful, appetite-whetting opener for a multicourse meal.