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.
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.)
Remove from heat and puree soup in batches in blender or directly in pot with hand-held blender.
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.