1/2 cup (1 stick)very cold unsalted butter, cut into 1/4'' squares
For the crumble topping:
1/3 cupgranulated sugar
1/2 cupall-purpose flour
1/4 teaspoonkosher salt
4tablespoons (1/2 stick)unsalted butter, cut into 1/2'' cubes
For the filling:
4cupsblackberries, fresh or frozen
2cupsblueberries, fresh or frozen
3/4 cupgranulated sugar
1tablespoonfresh lemon juice
For the egg wash:
1whole egg, lightly beaten with a fork
1 teaspoon water
For the pie crust:
In a large bowl, stir together the flour, salt, and sugar with a whisk. Add the butter squares and use your fingers to smear the butter into the flour mixture. Continue to incorporate the butter into the flour until the pieces of butter varied between the size of peas and oat flakes.
Sprinkle 3 tablespoons of the buttermilk into the flour and butter mixture. Use your hands to quickly incorporate the buttermilk. Add the 4th tablespoon of buttermilk as needed and incorporate until the mixture forms a shaggy dough that holds together somewhat.
Turn the dough out onto a well-floured surface and gently knead it into a disk that holds together. Wrap the disk in plastic wrap and refrigerate for at least one hour and up to overnight.
For the crumble topping:
Use a whisk to combine the sugar, flour, cinnamon, and salt in a medium bowl. Add the butter to the flour mixture. Use your fingers to smear the butter into the flour until the mixture looks like course, irregular crumbs. Refrigerate until ready to use.
For the pie filling and assembly:
Preheat the oven to 375 degrees F and place an oven rack in the lowest position.
Combine the berries, sugar, cornstarch, lemon juice, and salt in a medium bowl with a rubber spatula. Refrigerate the filling while you roll out the pie dough.
On a well-floured surface, roll out the pie dough to 1/8'' thick. Make sure you can fit your pie plate on top of the rolled dough with about 1/2-1'' of a border around the diameter of the pie plate. Place the rolled dough into the pie plate and crimp with your fingers or a fork.
Add the prepared fruit filling to the pie crust. Distribute the crumble topping evenly over the fruit.
Combine the 1 teaspoon of water with the beaten egg. Use a pastry brush to lightly sweep the egg wash around the edge/rim of the pie crust.
Place the pie on a baking sheet and bake for 60-80 minutes or until the filling is bubbling and the crust is golden brown. Use a crust shield as needed to keep the crust edges from browning too much (I typically put mine on at the 50-minute mark).
Remove the pie from the oven and allow it to cool on a cooling rack for 2-3 hours. The filling will continue to thicken and set as the pie cools. Slice when cooled - a scoop of vanilla ice cream on top is HIGHLY recommended!
This pie can be frozen (unbaked) for up to three months and baked in the frozen state. Baking from frozen may take an extra 20-30 minutes!