Preheat the oven to 400°F and line a baking sheet with parchment paper.
Place the squash on a sturdy cutting board. Using a sharp chef’s knife, slice the top stem off the squash and then cut the acorn squash in half lengthwise. Using a spoon, scrape out the seeds, then place the squash cut-side down on the baking sheet and transfer to the oven to roast for 30-40 minutes, depending on the size of the squash, until tender or a knife can easily pierce into the flesh. Once roasted, remove the baking sheet from the oven and set aside for a couple of minutes until the squash is cool enough to handle.
While the squash is cooling, in a large pot on medium heat, warm the butter and olive oil. Add onion, celery, and carrots and cook for 5-7 minutes until tender. Add garlic and cook for another 1-2 minutes until tender.
Add the nutmeg, cinnamon, and salt, stir to combine with the vegetable mixture, and cook for another minute until fragrant.
Once the squash is cool enough to handle, using a spoon, scrape the cooked flesh out of the skin into the pot, and stir to combine with the vegetable mixture.
Cover with broth, stir to combine, and bring mixture to a boil, then reduce to a simmer and allow to cook for 20 minutes.
Once cooked, using an immersion blender or in batches using a bar blender, purée the soup until smooth. If you find the soup too thick just add a splash of broth or water until you’ve reached the desired texture. Taste and season with additional salt and pepper as needed.
Once blended, the soup can be served immediately or cooled and stored in an airtight container in the fridge for up to 5 days or in the freezer for up to 3 months.