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1
To make the crust: In a large bowl, mix the flours, salt, and sugar together. Add in the butter and quickly rub it into the flour mixture with your fingers. You want the butter to break up into small pieces ranging from the size of peas to lima beans.
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2
Combine the water and apple cider vinegar in a measuring cup. Make a well in the flour/butter mixture and slowly stream 1/2 cup of water into the dough while mixing gently. Mix until the water is evenly distributed and the dough holds together when you squeeze it. It may look dry, and that's okay, just as long as it holds together when you squeeze it. If it is too dry, add more water, one tablespoon at a time. Chill the dough for one hour.
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3
After it has chilled, unwrap the dough and place it onto a lightly floured board. Pat the dough into a rough square, then roll it into an 8-inch x 11-inch rectangle. The dough will be a bit rough and crumbly and that's okay! With the long side of the dough facing you, gently fold the dough into thirds. Then turn the dough so the seam is at the top and parallel to your body. Repeat this process 2 more times then divide the dough in half, wrap each piece in plastic, and chill for at least 2 hours and up to 2 days before using.
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4
To make the pie: When you're ready to make the filling and assemble the pie, preheat oven to 400u00b0 F.
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5
On a lightly floured surface, roll out one piece of the dough into a 12-inch (approximately) circle about 1/8-inch thick and place it into a 9 or 10 inch pie pan. Place in the fridge while you prepare the rest of the pie.
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6
Roll out the other piece of dough into a rough square, about 1/8-inch thick, and place it on a parchment-lined sheet pan in the fridge to chill. Prepare the filling while the dough chills.
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7
In a large bowl, rub the lemon zest and vanilla bean seeds into the sugar, then add the peaches, blueberries, flour, nutmeg, and lemon juice. Toss gently to combine. If the mixture looks extremely juicy, add in the extra tablespoon of flour. (Reserve the egg and coarse sugar to finish the pie.)
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8
Cut the square of chilled dough into 8 to 10 strips using a pastry cutter, pizza wheel, or long knife. I like strips that are about 1-inch wide, but you can make them a bit smaller -- and they definitely don't have to be perfect.
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9
To make a lattice top, starting from the top left corner of the pie, lay one strip of dough horizontally and one strip vertically, so that the horizontal strip is on the bottom. Lay a second horizontal strip about 1/2 inch below the top strip so that it overlaps the vertical strip. Fold the top horizontal strip to the left and lay another vertical strip about 1/2 inch to the right of the first one. Next, fold the top horizontal strip back over it. This way, the second vertical strip will go underneath the top horizontal strip and over top of the second horizontal strip. Fold the first vertical strip up and lay down a third horizontal strip. Fold the vertical strip back down. Next, fold the second horizontal strip to the left and place a third vertical strip. Fold the horizontal strip back over the pie. Fold the middle vertical strip up and lay down the fourth horizontal strip. Fold the top and third horizontal strip to the left and place a fourth vertical strip. Continue until all strips are used and the top of the pie is covered.
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10
Trim the edges of the dough evenly, then fold the bottom dough up and over the lattice strips and crimp the edges of the dough. Slide the whole pie into the fridge or freezer for about 15 minutes before you bake it to firm up the crust. When you are ready to bake, carefully and gently brush the top of the pie with a beaten egg and sprinkle with a healthy dose of coarse sugar.
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11
Put the pie on a baking sheet to catch any drips and bake for 15 minutes on the lowest rack of your oven, then lower the oven temperature to 375u00b0 F and bake for 35 to 45 minutes, or until the crust is deep golden brown and the juices bubble. Cool the pie completely before serving.
