Melt butter in a large saucepan; add mushrooms and onion, and saute until tender. Stir in flour and curry powder; gradually add chicken broth, and cook over medium heat, stirring constantly, until mixture is thickened.
Stir in pumpkin and next 4 ingredients; reduce heat, and simmer, stirring occasionally, 10 minutes. Stir in milk, and cook, stirring constantly, until thoroughly heated. Garnish, if desired.