In large saucepan, heat oil over medium-high heat. Add onion and pumpkin-pie spice, and cook until onion is soft, about 6 minutes. Stir in beans and broth, and bring to a boil; reduce heat and simmer 5 minutes. Cool soup 5 minutes, puree in blender in two batches, and return soup to saucepan.
Bring soup to a boil, and stir in diced sweet potato; reduce heat and simmer until sweet potato is tender, about 10 minutes. Garnish with sour cream.