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Remove tart from baking sheet and let pan cool slightly on a rack. Remove from freezer, place tart pan on a baking sheet, and bake for about 20 minutes until golden. Pierce crust all over with a fork and freeze for 30 minutes. Press the dough evenly over the bottom and up the sides of the pan, then trim it by rolling your rolling pin over the top of the tart pan edge. Roll out dough on a lightly floured surface to a thickness of 1/8 inch, then transfer it into a 9 1/2-inch tart pan with a removable bottom.Form the dough into a disc, wrap in plastic and chill for at least 1 hour. Add butter and pulse until the mixture resembles coarse bread crumbs, then add egg yolks and pulse until the dough comes together. In the bowl of a food processor, place flour, confectioners’ sugar and salt, and quickly pulse to combine.Let it sit on the rack just to shake off this initial soft stage and to recrisp and refirm, which it will.) Cut into wedges, and serve while hot. (In the first few minutes straight out of the oven, the dough is kind of soft from the heat, possibly giving you the false impression that you have a soggy tart. Remove from the ring, and allow to cool just a few minutes on the rack, so that the piping hot tart shell can kind of tighten up enough to be sliced with a sharp chef’s knife.Return tart to oven on the sheet, and bake for 25 minutes, or until custard has set, the tops of the onions start to achieve a deeper brown and the dough is dark golden brown at the edges. Remove weights, fill with the onion-custard mixture and distribute it evenly. Remove tart shell from oven, and slip it onto a baking sheet.Stir well, and make sure the onions are all evenly coated with the custard. Season with pepper, nutmeg and salt to taste. In a bowl, beat the egg with the cream.Dock the bottom of the dough with the tines of a fork, weight the pastry with beans or weight and blind-bake for 25 minutes. Use your fingers to press the edges into the flutes, accentuating the shape of the dough edge. Lay dough into the pan, gently pressing into the bottom, and roll the pin across the pan to cut off the excess dough. Roll tart dough out to a 1/4-inch-thick round, and drape over a round 10-inch fluted false-bottom tart pan.This is achieved by a slow caramelization. Take all the minutes you need - 25 or so - to let them soften to translucent, then to let the water they release start to evaporate, then to allow the sugars they contain to start to brown in the pan, so that you end up with soft, sweet and evenly browned onions. In a wide sauté pan over medium-low heat, melt the bacon fat and slowly sweat the onions until they are caramelized.Slice the halves with the ribs (root end to sprout end direction), not against, to create julienne slices rather than half moons. Meanwhile, cut the onions in half and peel them.Be firm and brisk and get the dough past its shaggy stage into a neat disk, trying to avoid using your hands or too much kneading. Using a flexible plastic dough scraper instead of your warm hands, bring the dough together by folding and pressing.Make a well in the center and pour ice-cold water into the well.
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Dump butter-flour mixture into a medium stainless bowl.Scatter butter over flour, top with lid and pulse 12 pulses to cut butter into flour to a coarse meal consistency. Blend flour and salt in the bowl of a food processor.
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