Sour cream will break down and curdle if it is heated too long or becomes too hot. After adding the cold sour cream, cook the bisque just long enough for it to reheat. If you use reduced-fat sour cream, add it the same way.
For a delightful taste treat, serve this bisque thoroughly chilled. Garnish with thin slices of Granny Smith apples and a sprinkling of finely chopped crystallized ginger.
Spoon a dollop of sour cream on top of each serving, then swirl slightly into the soup with the tip of a knife. For a touch of crunch and color, sprinkle with poppy seed or chopped pecans.