Special equipment: an 8-inch square baking dish (preferably glass); a mandoline or sharp knife
For the crust: Pulse the flour, sugar and salt in a food processor to combine. Add 1/3 of the cubed butter, and pulse until combined. Add the remaining butter, and pulse until the mixture resembles coarse meal. Add the vinegar, then gradually add 1/4 cup ice water, pulsing until evenly combined. Squeeze the dough between your fingers; if it doesn't hold its shape, add 1 to 2 tablespoons more water. Turn the dough out onto a flour-dusted surface, and press into a ball. Break off 1/3 of the dough and form into a ball; form the remaining piece into a second ball. Wrap both in plastic and flatten into 1-inch-thick squares. Refrigerate at least 1 hour.
For the filling: Meanwhile, peel the potatoes, and slice 1/4 inch thick on a mandoline or with a sharp knife. Cook the bacon in a medium pot over medium heat, stirring occasionally, until crisp, about 10 minutes; remove with a slotted spoon, and let drain on a paper towel. Discard all but 2 tablespoons of the bacon fat. Cook the onions in the pot, scraping up any brown bits from the bottom, until softened and translucent, 3 to 4 minutes. Stir in the flour; when it coats the onions, gradually whisk in the half-and-half. Add the potatoes and salt, and bring to a boil. Lower the heat to medium-high, and cook, stirring, until the mixture has thickened and the potatoes are softened just enough to pierce with a fork but offer some resistance, about 10 minutes. (They will continue to cook in the oven.) Remove the pot from the heat, and stir in the sour cream. Season with 1 teaspoon more salt and several grinds of pepper. Set aside to cool while you prepare the crust.
Position an oven rack in the center of the oven, and preheat to 425 degrees F. Spray an 8-inch square baking dish with cooking spray.
To assemble: Lightly dust a work surface with flour. Roll the larger piece of dough into a 12-inch square about 1/8 inch thick. Transfer the dough to the prepared baking dish, pressing it into the corners as necessary. (Leave any overhang for pressing together with the top crust later.) Sprinkle the breadcrumbs over the dough. Top with the potato mixture, and shake the pan to distribute evenly. Sprinkle the crispy bacon on top.
Roll the remaining dough to 1/8 inch thick, and place it on top of the pie. Press it together with the overhanging bottom crust with your fingers to seal the edges, and trim to 1 inch around the dish. Roll the dough under, and press it into the top of the dish; crimp the edges with your fingers, or press with a fork. Cut slits into the top of the dough to allow steam to escape.
Bake the pie on a rimmed baking sheet on the center rack for 30 minutes, then rotate the pie to allow even browning. Continue to bake until the dough is nicely browned, especially on the bottom and sides, and the potatoes are tender when pierced with a knife, 30 to 40 minutes more. Let cool for 25 minutes before cutting.
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