How to Make It
In a large saucepan, melt 2 tablespoons of the butter. Add the onion and cook over moderate heat, stirring occasionally, until softened but not browned, about 5 minutes. Add the pumpkin, stock and nutmeg and bring to a boil. Cover and cook over moderately low heat until the pumpkin is tender, about 30 minutes. In batches, puree the soup in a blender until smooth. Return the soup to the saucepan, season with salt and pepper and keep warm.
Meanwhile, in a large skillet, melt the remaining 2 tablespoons of butter. Add the mushrooms and cook over moderately high heat, stirring occasionally, until tender, about 3 minutes. Add the shallot and cook, stirring, until softened, about 2 minutes. Season with salt and pepper and stir in the chopped parsley.
Ladle the soup into bowls. Top with a dollop of sour cream and some of the mushrooms. Garnish with parsley and serve.