Prep the oven and pan. Heat the oven to 425°F. Line a large rimmed baking sheet with foil.
Season and roast the sweet potatoes. 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 30-40 minutes, or until tender.
Sauté the aromatics. Meanwhile, in a large saucepan, saute the onion and celery in the remaining 1 tablespoon of olive oil until the onion is translucent. Add the garlic and saute for 2 minutes longer.
Add the broth and seasonings. 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. Add the sweet potatoes to the saucepan and continue cooking until the vegetables are all very tender, about 5 to 10 minutes longer.
Blend the soup. Blend the hot mixture in 3 to 4 batches in the blender until smooth. Return the soup to the saucepan and stir in the cream. Heat through. Taste and adjust the seasonings.
Garnish. Serve with a garnish of fried sage leaves, chopped chives, a drizzle of maple syrup, or chopped toasted pecans.