- Preheat oven to 350°F.
- Slice sweet potatoes in half lengthwise; drizzle cut surfaces lightly with olive oil. Place cut side down on a baking sheet and roast uncovered for 1 hour or until tender and fully cooked. Cool. Scoop tender sweet potato out of skins and rough chop; reserve.
- Melt butter in a heavy soup pot over medium heat; add shallots and saute for 2 minutes or until tender. Add flour and stir over medium-low heat for 2 minutes. Slowly add chicken broth and whisk until smooth. Add reserved sweet potatoes, brown sugar, cinnamon nutmeg and salt. Simmer over low heat 10 minutes or until completely heated through.
- Working in several smaller batches, carefully puree bisque in a food processor fitted with a steel blade. Return to soup pot; add half and half and simmer over low heat for 5 minutes or until fully heated. Taste and adjust seasonings.
- To serve, ladle bisque into soup bowls. Garnish each with a drizzle of crème fraiche and a sprinkle of candied pecans.