Preheat the oven to 400 degrees F and line a large baking sheet with aluminum foil or parchment paper. Place the butternut squash on the sheet and toss with a thin coating of olive oil (about 1 tablespoon). Spread in a single layer and sprinkle lightly with salt. Roast the squash for 35-45 minutes, or until fork tender. (Note: if you want to roast a whole butternut squash instead, see the recipe notes for steps).
In a large, nonstick heavy-bottom saucepan or Dutch oven, heat 1 tablespoon olive oil over medium-low heat. Add the onions and celery along with a pinch of salt and cook, stirring frequently, for about 10 minutes, until the vegetables are soft and starting to slightly caramelize. Add the garlic and cook for an additional minute, stirring.
Turn up the heat to medium and continue stirring until a brown glaze begins forming on the bottom of the pot, 1-3 minutes. If you're using a nonstick pan, there won't be any brown bits (which is fine).
Add the white wine. The liquid will help detach the brown bits; use a spatula to scrape them up and stir them back into the veggies. (If you don’t use alcohol, water or stock may be substituted.)
Add the squash, stock, thyme, cinnamon, and nutmeg. Turn the heat to high and bring the soup to a boil, then reduce to low and cover. Simmer for 30 minutes.
Remove the soup from the heat and let it cool, uncovered, for 5 minutes. Remove the thyme stems (the leaves will be in the soup). Carefully ladle the soup into a blender and puree, in batches, if necessary.
Return the soup to the pot and season with salt and pepper. As with most soups, this one will taste best the next day after the flavors have had a chance to truly mingle. You can reheat the over medium heat in the same saucepan, or in smaller batches in the microwave. The soup will keep in the refrigerator for 3-5 days. It can also be frozen in airtight containers for several months.
Serve soup in bowls topped with spiced pistachios and some freshly ground black pepper.
Prepare the spiced nuts:
Preheat the oven to 350 degrees F and line a baking sheet with aluminum foil.
Place the pistachios in a small bowl and add the olive oil, salt, brown sugar, cinnamon, chili powder, and cayenne pepper, stirring with a spatula to combine.
Spread the nuts in a single layer on the prepared baking sheet, and roast for approximately 15 minutes, shaking the pan halfway through. The pistachios are ready when they’re golden and fragrant. They can be prepared up to one week in advance and stored in an airtight container at room temperature.
You can see step-by-step photos for how to dice and roast butternut squash in this tutorial. To save time, you can use pre-diced squash from the produce section of your grocery store, or you can roast the squash whole.Roasted Whole Butternut Squash:
Cut a small slice off the bottom of the squash to create a flat surface, then place the squash flat against a cutting board. Slice in half from top to bottom (use a sharp chef's knife).
Scoop out the seeds, then rub the squash with a bit of olive oil. Place on a parchment-lined sheet pan, flesh-side down, and roast in a 400 degree F oven until fork tender, 45-60 minutes.
If you can't find raw pistachios, the roasted ones will still work, but try to avoid salted pistachios. If that’s your only option, omit the salt from the spice mixture.If you want to make a pumpkin soup or acorn squash soup instead, you can swap in equal quantities in place of the butternut squash.