In a large pan (dutch oven) heat oil--add onion, celery, garlic, ginger and sugar and cook 4 minutes until tender. Add salt, allspice and nutmeg; stir to coat. Add pumpkin, potato and broth and bring to a boil. Reduce heat, partially cover and simmer 20 minutes or until pumpkin is tender.
Puree half the soup in a blender and return to pan to heat.
Can be made ahead and frozen. Thaw in refrigerator a couple days ahead. Reheat approx. 1/2 hour before serving.
Serve topped with sour cream and onions.