Roast sweet potatoes in 450 degree F (230 degrees C) oven for 30 minutes. Remove from oven and slice open, allowing to cool while assembling rest of soup.
2
Saute onion and garlic in olive oil and butter until soft and slightly browned. Add potatoes to pot and saute 1 minute. Stir in flour. Deglaze pan with broth, scraping up brown bits. Add water and sweet potato flesh. Bring mixture to a boil and reduce heat to medium low. Simmer partially covered for approximately 30 minutes or until vegetables are tender. (Liquid will have reduced to approximately 8 cups.)
3
Remove from heat and puree soup in batches in blender or directly in pot with hand-held blender.
4
Return soup to pot and re-heat over low heat. Add buttermilk, nutmeg, ginger, white pepper, thyme, cumin, and salt and pepper to taste, stirring well.
