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4 tablespoons butter
1/4 cup onion, chopped fine
1 large butternut squash (about 3 pounds), cut in half lengthwise, and each half cut in half widthwise; seeds and strings scraped out and reserved (about ¼ cup)
6 cups water
1 teaspoon kosher salt
1/2 cup heavy cream
2–4 tablespoons brown sugar (depending on your taste buds)
Pinch of grated nutmeg

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Melt the butter in a large Dutch oven (or heavy pot) over medium-low heat until foaming. Add the onion and cook, stirring frequently, until translucent, about 3 minutes. Add the squash scrapings and seeds, and cook, stirring occasionally,for 3 to 4 minutes.
Add the water and 1 teaspoon salt to the pot and bring to a boil over high heat. Reduce the heat to medium-low, place the squash cut-side down in a steamer basket, and lower the basket into the pot. Cover and steam until the squash is completely tender, about 30 minutes. Remove the pot from the heat, and use tongs to transfer the squash to a rimmed baking sheet. When cool enough to handle, scrape the flesh from the skin using a large spoon. Reserve the squash flesh in a bowl and discard the skins.
Strain the steaming liquid through a mesh strainer into a second bowl; discard the solids in the strainer. (You should have 2½ to 3 cups of liquid.) Rise and dry the pot.
Puree the squash in batches in a blender or food processor, pulsing and adding enough reserved steaming liquid to obtain a smooth consistency. Transfer the puree to the pot and stir in the remaining steaming liquid, cream, and brown sugar. Warm the soup over medium-low heat until hot, about 3 minutes.
Stir in the nutmeg and season with salt to taste. Serve immediately. Soup can be refrigerated in an airtight container for several days. Warm over low heat until hot, but don’t boil.

I don’t have a steamer basket, so I used the steamer insert from my rice cooker. Use whatever you have that will work.