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Whisk together the flour/s and salt in a large, sturdy bowl. Cube the cold butter and add it to the flour. Using your fingers or a pastry blender, break up the butter into the flour until the chunks of butter are no larger than a pea.
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In a separate bowl, whisk together the sour cream or yogurt, cider vinegar and ice water. Add it to the flour and use a silicone spatula to stir together. Dump it onto a clean work surface and fold or knead a few times (do NOT overwork) just to bring it together--I needed to sprinkle just a few extra drops of water to get it to come together as it was quite dry here when I made it.
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Wrap the dough in plastic wrap and place it into the fridge to chill for at least an hour or even overnight.
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About 90 minutes before dinner time, start preparing the filling. First preheat the oven to 400 F. Line a cookie sheet with foil.
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Slice the butternut squash lengthwise and scoop out the seeds. Place the 2 halves onto the cookie sheet and drizzle 1-2 tablespoons of olive oil over the squash. Sprinkle with fresh ground black pepper and coarse salt, about 1/2 teaspoon of each. Place in the oven to roast until fork-tender, about 45 minutes (start checking at 30 minutes, depending on the squash and the oven it could take less time).
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While the squash is cooking, slice the onions. Heat a skillet with a tablespoon of oil on medium high heat and add the onions with a pinch of salt. Toss occasionally.
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While keeping an eye on the onions, heat another skillet with a tablespoon of olive oil on medium high heat and add the mushrooms with a pinch of salt. Toss the mushrooms occasionally as well.
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In the meantime, remove the butternut squash when it is fork tender and let it cool a bit (you want it cool enough to handle). Leave the oven on 400 F.
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Reduce the heat on the onions and mushrooms both to medium or medium-low--you do not want to have to worry about them scorching while you multitask.
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Keeping an eye on your onions and mushrooms, tossing them occasionally, scoop the butternut squash out of its skin and add it to a large bowl or the bowl of a stand mixer. Add the goat cheese and thyme and beat until completely incorporated.
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When the mushrooms have released most of their water, add the sherry and stir, scraping any stuck bits from the pan.
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When the sherry has evaporated and the onions have caramelized to a rich brown (about 15 minutes), add a splash of balsamic vinegar with a pinch of salt to each pan, about a tablespoon or a little over that. No need to use a fancy balsamic here. Use the vinegar in both pans to deglaze and scrape up the bottom of the pan. Let the liquid mostly cook off and then turn off the heat for both pans.
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Add the onions to the squash-goat cheese mixture, and mix in. Taste for salt and pepper, and add what is needed. Set aside.