A great soup for an elegant summer dinner. The recipe is based on a Swedish dish for which lingonberries normally would be used. If you can find any, use them. Otherwise, cranberries make a fine American adaptation to this dish. This soup keeps very well, and leftovers are wonderful stirred into plain or vanilla yogurt.
In a medium-size saucepan, combine all the ingredients except the cranberry sauce and sour cream. Over medium-high heat, bring the mixture to a boil, stirring constantly; then reduce the heat and simmer, stirring constantly, for 5 minutes. Remove from the heat and stir in the cranberry sauce. Cover and refrigerate until well chilled. Serve cold, with a dollop of sour cream.