3 pounds butternut squash – halved lengthwise and seeded
2 tbsp olive oil
1.25 tsp kosher salt – divided
0.5 tsp black pepper – divided
1 medium onion – chopped
2 cloves garlic – sliced
1 tbsp unsalted butter
1 tsp fresh thyme – leaves stripped
0.25 tsp ground nutmeg
4.5 cups vegetable broth – divided
0.25 cups heavy cream
pumpkin seeds – toasted (for serving)
crème fraîche – for serving
sour cream – for serving
chives – finely chopped (for serving)
Instructions
Heat the oven to 425°F. Line a rimmed baking sheet with parchment.
Place the butternut squash halves cut-sides up on the sheet. Rub with the olive oil and season with about 0.75 tsp of the kosher salt and 0.25 tsp of the black pepper. Turn cut-sides down.
Roast until the flesh is very tender and the edges are caramelized, 35–45 minutes. Cool just enough to handle.
Meanwhile, melt the unsalted butter in a large pot over medium heat. Add the chopped onion and 0.25 tsp of the kosher salt and cook, stirring, until translucent and sweet, 6–8 minutes.
Stir in the sliced garlic, fresh thyme leaves, and ground nutmeg; cook until fragrant, 30–45 seconds.
Scoop the roasted squash flesh from the skins and add it to the pot. Stir to combine.
Pour in 4 cups of the vegetable broth (reserve 0.5 cup) and bring to a gentle simmer. Cook 8–10 minutes to meld flavors.
Blend the soup until completely smooth using an immersion blender directly in the pot. (Or blend carefully in batches in a countertop blender, venting the lid and covering with a towel.)
Adjust consistency with up to the remaining 0.5 cup hot vegetable broth until velvety and pourable.
Stir in the heavy cream and warm gently over low heat without boiling, 1–2 minutes.
Season with the remaining 0.25 tsp kosher salt and 0.25 tsp black pepper, tasting and adjusting as needed.
Ladle into bowls and garnish with crème fraîche or sour cream, toasted pumpkin seeds, and chopped chives (optional). Serve hot.