Prepare the Pie Dough (Morning Before Serving or Prepped the Day Before): Combine the all-purpose flour, sugar, and salt in the bowl of a stand mixer. Note: If you do not own a stand mixer, you can prepare the dough in a large food processor or by hand using a pastry cutter.
Using the paddle attachment, mix the dry ingredients for 5 to 10 seconds over low speed. Over low speed, slowly add the cold cubed unsalted butter, a few pieces at a time, until the butter has broken into a mixture of larger and smaller size pieces (similar to ‘broken walnut pieces’). Add the vinegar and slowly add the ice water (you will most likely need to use all of it or a touch more, but this will depend on humidity) until the dough just barely holds together. It will look relatively dry and shaggy in the bowl, but if the dough can hold together when squeezed with your palm, it is ready.
Transfer the pie to a large gallon-size Ziplock bag (this helps lessen the warmth from your hands from transferring to the dough) and using the open ends and sides of the bag, press the dough together into a ball using the heels of your hand.
Remove the pie dough from the bag and using a sharp knife, cut the dough into two equal-sized pieces. Flatten each piece into a round disc and wrap tightly in plastic wrap. Refrigerate for at least 2 hours or, ideally, overnight.
Blanch the Peaches, Shape Pie Dough, and Preheat the Oven: Preheat the oven to 425°F (220°C) with a rack in the center position. Line a half sheet pan with parchment paper or foil. Set aside.
Bring a large pot of water to a boil. Set aside a large ice bath near the stove. Using a paring knife, make a small shallow ‘X’ incision onto the bottom of each peach and carefully drop into the boiling water. Allow the peaches to blanch for roughly a minute (depending on the ripeness of your peaches) or until the skins begin to slightly peel away from the flesh. Using a skimmer or large spoon, transfer the peaches to the ice bath (this will prevent them from cooking any further) until cool enough to handle. Transfer to a paper towel-lined plate and set aside.
Meanwhile, remove one disc of chilled pie dough from the fridge. Allow it to sit at room temperature for 5 to 10 minutes. On a well-floured countertop and using a lightly floured rolling pin, roll the dough until it roughly a 12-inch disc that is roughly ⅛-inch thick. Lift and transfer to a 9-inch pie pan (equipment note: I recommend heavy ceramic) pressing it into the edges of the pan without stretching the dough. Using a knife or kitchen scissors, trim any remaining dough overhang to just one inch. Brush off any excess flour with a pastry brush and place the pie pan in the freezer for 10 minutes or cover tightly with plastic wrap and refrigerate for up to 30 minutes. Remove the remaining pie dough disc from the fridge, unwrap, and roll into a 12-inch diameter disc that is roughly ⅛-inch thick or less. Place the dough between two sheets of plastic wrap or parchment paper and chill in the fridge while you prepare the pie filling.
Prepare the Pie Filling: Using a pairing knife, carefully peel the skin from the blanched peaches, remove the pits, and cut into ½-inch thick wedges. Transfer the peach wedges to a large mixing bowl and add the blackberries, brown sugar, granulated sugar, cornstarch, lemon juice, cinnamon, nutmeg, and salt. Toss gently to combine.
Remove the pie pan from the freezer and add the peach blackberry filling. Remove the rolled top crust pie dough from the refrigerator and place over the fruit filling, tucking the overhang of the top crust over and then under the bottom crust overhang. Press and crimp the edges of the crust as desired.
Using a sharp paring knife, cut several 2-inch slashes into the top crust. Set the assembled pie in the freezer to chill for 20 minutes (or alternatively, cover with plastic wrap and refrigerate for up to 1 hour). This will allow the pie dough to relax, the butter to chill, and helps prevent shrinking during baking.
Bake: Brush the top crust with heavy cream and sprinkle with sparkling sugar and set the pan on top of the lined sheet pan. Bake for 15 minutes. Reduce the oven temperature to 400°F (205°C) and bake for an additional 15 to 20 minutes or until the pie crust is lightly golden. Reduce the heat to 350°F (175°C) and continue to bake until deeply golden brown, an additional 25 to 35 minute. The filling should be lightly bubbling. Transfer the pie to a wire rack and allow to cool for at least 2 to 3 hours before slicing. Serve slightly warm or at room temperature with vanilla ice cream or whipped cream.
Tips for Success:
It is essential that your pie dough ingredients start/stay as chilled as possible! This ensures minimal gluten development (tenderness) and allows the butter to remain cold as it hits the hot oven (producing flaky layers). Butter that starts to melt or soften will produce a tougher crust with very little flakiness.
A small addition of white vinegar (aka. acid) helps discourage additional gluten formation as well. If you do not have white vinegar on hand, you can substitute it with apple cider vinegar or freshly squeezed lemon juice for a similar effect. It is not an essential ingredient by any means, but doesn’t hurt!