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1
Heat the oven to 450 and line two baking sheets with foil.
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2
Trim the top and bottom from the squash and peel it, using a very sharp vegetable peeler or a paring knife. Slice it in half lengthwise, remove the seeds and stringy insides, and then slice it crosswise 1/3-inch thick. On one of the prepared baking sheets, toss the squash with 1 tablespoon of the olive oil, 1/2 teaspoon salt, and the thyme, then spread it into a single layer. On the other prepared baking sheet, toss the onion with the remaining oil and 1/2 teaspoon salt, and spread it into a single layer. Roast the vegetables until the squash is very tender and browning, and the onion is soft and brown--around 15 minutes for the onion, and 25-30 for the squash. Toss the onion with the balsamic vinegar, and turn the oven down to 400.
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3
While the vegetables cook prepare the dough: in the bowl of a food processor fitted with the steel blade, pulse the flour with 1/2 teaspoon salt, then add the butter slices and pulse until the mixture just forms coarse crumbs. Whisk together the sour cream and water, pour the mixture through the feed tube, and pulse again until the dough just starts to come together into large clumps. Turn the dough onto a work surface and gather it gently together, then press it into a disc, wrap it in plastic wrap, and chill it in the freezer while the vegetables finish cooking.
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4
When you are ready to assemble the tart, roll the floured dough out on a floured surface until it is a thin 12-inch circle (it doesn't need to be perfectly thin or a perfect circle!). Fold the dough loosely into quarters to transfer it to an ungreased baking sheet (simply removed the foil and veggies from one you've already used), then unfold it back into a circle. Arrange the squash over the dough, leaving a 1- or 2-inch border around the edge which you will fold over later. Arrange the cheese and pecans over the squash and then, finally, top with the onions and any of their vinegary juices that have accumulated in the pan. Fold the border over the filling, pleating as necessary to keep its shape; the center will be open.
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5
Bake at 400 for 30 to 40 minutes until deeply golden. Cool briefly, then cut into wedges and serve warm or at room temperature.
