If you’d like a thicker, juicier filling, add 2 more cups of blueberries and another tablespoon of cornstarch. And make sure to set a sheet of foil or a baking sheet on the bottom of your oven to catch any spillage.
Prepare the dough: In a liquid measuring cup, combine the ice water and vinegar.
In a large bowl, whisk the flour with the almond meal, sugar and salt. Add the butter, and use a pastry blender or your fingers to work it into the flour until most of it is the size of peas, with a few larger chunks remaining. Mix in the vinegar mixture, tablespoon tablespoon at a time, until the dough just starts to hold together with a few dry spots remaining. Transfer half the dough to a large sheet of plastic wrap and gently knead it to bring in any ragged edges. Pat the dough into a rectangle, wrap it in plastic and refrigerate for at least 1 hour or for up to 3 days. Repeat with the remaining dough.
Assemble and bake the pie: Remove one piece of dough from the refrigerator and roll it out between 2 floured sheets of plastic wrap or parchment paper until you have an 18- by 13-inch rectangle. If the dough starts feeling too soft as you’re rolling, refrigerate it for 15 to 20 minutes Fit the dough into a 15- by 10-inch baking sheet, leaving any excess intact, and refrigerate. In a bowl, toss the peaches with the blueberries, sugar, cornstarch, lemon juice and salt.
Roll out the second piece of dough to a 16- by 11-inch rectangle. Spread the filling out in the crust-lined pan. Drape the second piece of dough on top of the filling. Roll and pinch the excess crust inward to create an edge around the pan. Use a sharp knife to cut vents all over the top crust. Refrigerate the assembled pie for 20 minutes. Meanwhile, arrange a rack in the bottom of the oven and another in the center. Preheat the oven to 425°F. Brush the top pastry with the egg wash and sprinkle with the demerara sugar.
Bake the pie on the bottom rack of the oven for about 25 minutes. Reduce the temperature to 375°F and move the pie to the center rack. Bake for 30 to 35 more minutes, until the pastry is golden brown and the juices are bubbling. Let the pie cool on a wire rack for at least 2 hours before serving.