Preheat the oven to 425°F. Line a large baking sheet with parchment paper.

Place the squash halves cut-side up on the prepared baking sheet. Drizzle with olive oil and brush or rub to coat the flesh evenly. Season with some of the salt and a few grinds of black pepper, then turn the squash cut-side down.

Roast for 40 to 50 minutes, or until the squash is fork-tender. Let cool for about 10 minutes.

Meanwhile, heat the olive oil in a Dutch oven or large saucepan over medium heat. Add the onion and the remaining salt. Cook until softened, stirring occasionally.

Stir in the garlic and cook until fragrant, about 30 seconds. Remove from the heat.

Scoop the roasted squash from the skins and transfer it to a blender. Add the onion mixture, nutmeg, black pepper, and the vegetable broth.

Blend until completely smooth. If using a smaller blender, work in batches and avoid filling above the maximum fill line.

If you'd like a thinner soup, blend in additional broth until it reaches your desired consistency.

Add the butter and blend briefly until melted and incorporated. Taste and season with additional salt and pepper, if needed.

Return the soup to the saucepan and warm over medium-low heat, stirring occasionally, until heated through.

Ladle into bowls and, if desired, garnish with a drizzle of heavy cream and freshly ground black pepper.