Heat the oven to 425°F (220°C). Toss the butternut squash and carrot with the olive oil, spread them in a single layer on two rimmed baking sheets without crowding, and roast until pieces are well caramelized on a few sides, about 40 minutes, turning with a thin metal spatula two or three times during baking. Remove from oven and set aside.

Warm a large saucepan over medium-high and melt 2 tablespoons of the butter until it just starts to brown. Add the sliced onion and garlic, reduce the heat to medium, and cook, stirring occasionally, until the onion is soft and lightly golden, about 10 minutes; lower heat as needed to avoid browning too quickly.

Transfer the roasted squash and carrots into the saucepan and remove the pan from the heat. Pour 1/2 cup (120 ml) of stock onto each baking sheet, scrape up the browned bits, and add that liquid to the saucepan. Stir in the remaining stock, thyme sprigs, and bay leaf, return the pan to medium-high heat, bring to a simmer, then reduce the heat and simmer until the vegetables are very tender, about 10 minutes. Discard the thyme sprigs and bay leaf.

Carefully purée the soup until completely smooth using an immersion blender or transfer in batches to a high-powered blender. Season with salt and pepper to taste. Return the soup to the saucepan and thin with extra stock if you prefer a looser consistency; keep warm over low heat.

While the soup stays warm, whip the heavy cream and sour cream together with the cinnamon, nutmeg, and a pinch of salt using a stand mixer, handheld mixer, or whisk until firm peaks form. Set the spiced whipped cream aside.

If making frizzled sage, melt the remaining 5 tablespoons of butter in a small skillet over medium-high heat. Swirl the pan so the butter coats it, and when it is foamy and just beginning to brown, add the sage leaves and submerge them briefly, cooking about 15 seconds until crisp. Transfer the leaves with a slotted spoon to paper towels to drain and lightly season with salt.

To serve, ladle the hot soup into warmed bowls, add a dollop or two of the spiced whipped cream to each, and top with frizzled sage leaves if desired. Serve immediately.