Melt the butter in a soup pot. Add the onions and saute gently for about 10 minutes. Add the pumpkin, salt, pepper, brown sugar, nutmeg and allspice. Slowly add the broth. With an immersion blender, blend until smooth. (or in a blender). Heat thoroughly, then add cream. Heat again but do not boil. I like to garnish it with a dollop of sour cream and a little freshly ground nutmeg.