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.