Preheat the oven to 400°F. Place the squash on a rimmed baking sheet, drizzle over the melted butter, season with salt and pepper and toss to combine. Spread out in a single layer on the baking sheet, allowing space in between the cubes when possible. Roast for about 35 to 40 minutes until tender and lightly browned on the bottom.
Heat the oil in a stockpot over medium heat. Add the shallots or onions and sauté for 10 minutes, until the shallots have softened and turned a light golden brown. Add the broth and bring to a simmer.
Add the cooked butternut squash to the simmering broth and working in batches, puree the soup in a blender or food processor (alternately you can run the mixture through a food mill, which offers a great kind of nubby texture, or puree right in the pot with an immersion blender). Return the pureed soup to the pot. Taste and adjust the seasoning.
Add the cream and heat just until warmed through. Ladle the soup into bowls, and dollop or swirl in a bit of the sour cream or crème fraiche, and sprinkle with some pumpkin seeds, crisped bacon, or chives, if desired.
Why Do You Roast Squash Before Making Soup?
Roasting butternut squash before you puree it into a soup gives it that lovely caramelization. This deepens the natural sweetness of the squash and gives the soup a depthness of flavor that you wouldn’t get with steaming or boiling the squash.