What makes this soup so flavorful is that you roast the veggies in advance (squash, onions, apple). The roasting process concentrates and caramelizes the flavors. Then after roasting, it's a simple process to puree all the ingredients in a blender or processor. Just before serving, reheat and serve with croutons and a dollop of sour cream. We like this soup served the day after Thanksgiving with turkey sandwiches and a side dish of cranberry relish.