Special equipment: A large rimmed baking sheet and a pizza stone or 2 large rimmed baking sheets; a 1-inch star-shaped cookie cutter
For the dough: Combine the flour, granulated sugar and salt in a food processor and pulse until mixed. Add about one third of the chilled butter and process until thoroughly combined (the mixture will be slightly yellow). Add the remaining chilled butter and pulse until the mixture resembles a very coarse meal with pea-size bits of butter. Add the vinegar in a single pulse. Gradually add the ice water through the feed tube, pulsing just until evenly combined. Squeeze a handful of the dough together, it should just hold its shape and be a little crumbly. If still very powdery, pulse again, adding up to 2 tablespoons ice water.
Turn the dough out onto a work surface and press it together by hand. Divide the dough in half; wrap each piece in a large piece of plastic wrap, pressing each firmly into a 1/2-inch-thick rectangle. Refrigerate until firm, at least 1 hour or up to 2 days.
Adjust an oven rack to the bottom position of the oven and put a pizza stone or large baking sheet on it. Preheat the oven to 400 degrees F.
For the filling: Toss together the nectarines, brown sugar, lemon juice, cornstarch, vanilla and salt in a large bowl until the cornstarch dissolves. Add the raspberries and gently toss to combine.
To assemble the pie: Let the dough stand at room temperature for a few minutes until slightly softened to make rolling easier. Place one piece of dough between two large pieces of floured parchment. Roll into a rough 14-by-12 inch rectangle (if the dough gets too soft, pop it in the fridge to firm up, then continue rolling). Repeat rolling with the second piece of dough.
Pile the nectarine-raspberry filling into the center of one piece of dough and spread evenly, leaving at least a 1-inch border. Dot the filling with the butter.
With a 1-inch star-shaped cookie cutter, cut out 4 to 5 stars from the second piece of dough (reserve the cut-outs for decorating). To transport the dough to the pie, roll it up onto the rolling pin and then unroll it over the filling, lining up the edges with the bottom piece of dough as best you can. Pinch the edges together, then fold and crimp to create a tight seal. Brush the top and edges of the crust with the egg; decorate the top crust with the star cut-outs and brush them with egg too. Generously sprinkle the pie with granulated sugar. Using the bottom piece of parchment paper, slide the pie onto another rimmed baking sheet (trim the parchment if there's any overhang).
Put the baking sheet with the pie directly on the preheated pizza stone or baking sheet (this helps make the bottom crust crisp and flaky). Bake the pie until the crust is deep golden brown and the filling is bubbly, 45 to 50 minutes. (If the crust browns too quickly during baking, cover the pie loosely with foil. Some of the filling may bubble from the vents onto the parchment paper; don't fret-just embrace the rustic charm.)
Let cool. Slice into wedges and serve warm or at room temperature.
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