Roast sweet potatoes in a 230 C / Gas mark 8 oven for 30 minutes. Remove from oven and slice open, allowing to cool while assembling rest of soup.
Sauté onion and garlic in olive oil and butter until soft and slightly browned. Add potatoes to pot and sauté 1 minute. Stir in flour. Deglaze pan with stock, scraping up brown bits. Add water and sweet potato flesh. Bring mixture to the 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 2 litres.)
Remove from heat and purée soup in batches in blender or directly in pot with hand-held blender.
Return soup to pot and re-heat over low heat. Add remaining ingredients, stirring well.
Substitute double or single cream for the buttermilk if desired.
I switched the numbers for the potatoes and sweet potatoes so that there are more sweet potatoes. Cut the thyme and add 4 tbsp chopped coriander, 2 tsp paprika, 1/4 tsp. curry powder and 1/4 tsp. cinnamon. Garnish with coriander and soured cream instead of chives. Brilliant. This had a great colour and consistency. I expected a more earthy, filling quality, but it's even better the next morning. Use an immersion blender to purée it all. It's easier than transferring it to a traditional blender - 24 Jul 2008
by KAREN HUTCHISON
I made a few changes to start -- I increased the number of sweet potatoes to 4, and regular potatoes to about 2.5. Along with the onions and garlic, I sauteed about a 2 chopped carrots. I decreased the water by about 1/3, and made up the difference with chicken stock. After the spices were added, I also added 1 generous tablespoon cinnamon, and a full pint of buttermilk. While the spices do give this dish flavour, the sweet potato flavour is sorely lacking. I will likely up the number of sweet potatoes next time to 5 or 6, and definitely cut back on the amount of regular potatoes, nutmeg and thyme as they were a bit overpowering. - 24 Jul 2008