Preheat the oven to 450°F. Line a rimmed baking sheet with parchment paper.

Place the cut squash in a large bowl and toss with the olive oil, 1 teaspoon of the salt, and 1/2 teaspoon of the pepper. Transfer the squash to the prepared baking sheet and roast in the oven for 25 minutes, or until the squash is lightly caramelized and tender. Remove it from the oven and set aside.

While the squash is roasting, melt the butter in a 6-quart pot over medium heat. Add the onions, carrots, garlic, thyme, 2 teaspoons salt, and 1/2 teaspoon pepper. Cook until the vegetables are soft, about 10 minutes. Add the brandy, cook for 5 minutes, and then add the stock and water. Bring to a boil, reduce the heat, and simmer for 15 minutes.

Add the roasted squash to the pot, and remove it from the heat. Discard the thyme sprigs.

Blend the soup using an immersion blender, or in several batches in a blender until it is completely smooth. Transfer the blended soup to a decorative soup pot or to individual serving bowls. Serve hot, garnished with a dollop of crème fraîche and a drizzle of herb oil if desired.