Toss the diced sweet potatoes with 2 tablespoons of olive oil. Arrange the sweet potatoes on the prepared baking sheet and season with salt and pepper.
Roast the sweet potatoes for 30 to 40 minutes, or until tender.
Meanwhile, in a large saucepan, saute the onion and celery in the remaining 1 tablespoon of olive oil until onion is translucent. Add the garlic and saute for 2 minutes longer. Add the vegetable broth, the chopped sage, cinnamon, nutmeg, and brown sugar. Bring to a boil. Reduce heat to simmer for 10 minutes.
Add the roasted sweet potatoes to the saucepan and continue cooking until vegetables are all very tender, about 5 to 10 minutes longer.
Blend the hot mixture in 3 to 4 batches until smooth. Return to the saucepan and stir in the cream. Heat through. Season to taste with salt and pepper.
Serve with a garnish of fried sage leaves, chopped chives, a drizzling of maple syrup, or chopped toasted pecans.