1 cup cool (does not have to be ice-cold) unsalted butter, cut into pieces
1/4 cup cool water
2 teaspoons lemon juice or white vinegar
For the filling:
1 recipe Pie Dough for Fruit Fillings, chilled
6 cups peeled and sliced (1 centimeter thick), mixed apples (such as Mutsu, Granny Smith, Royal Gala, Honeycrisp, or Cortland) about 5 medium apples
1 tablespoon fresh lemon juice
1/2 cup granulated sugar
1/3 cup packed light brown sugar
1 teaspoon ground cinnamon
1/4 teaspoon ground allspice
1/4 teaspoon ground nutmeg
2 tablespoons rolled oats
2 tablespoons unsalted butter
1 egg, whisked with 2 tablespoons water, for brushing
Granulated sugar and ground cinnamon, for sprinkling
For the pie dough:
Stir the flour, sugar, and salt together in a bowl or in the bowl of a stand mixer fitted with the paddle attachment. Add the oil and cut it in using a pastry cutter (or blend on low speed) until the flour looks evenly crumbly in texture. Add the butter and cut in (or blend) again until rough and crumbly but small pieces of butter are still visible.
Stir the water with the lemon juice or vinegar and add this to the flour mixture all at once, mixing just until the dough comes together. Shape the dough into two discs (unless otherwise directed by the pie recipe you are following), wrap in plastic wrap, and chill until firm, at least two hours. The dough can be made up to two days ahead and stored in the fridge. Alternatively, it can be frozen for up to three months and thawed in the fridge before rolling.
For the filling:
Pull the dough from the fridge 15–30 minutes before you are ready to roll it. Preheat the oven to 400°F (200°C). Line a baking tray with parchment paper or aluminum foil.
Toss the sliced apples with the lemon juice in a large non-reactive bowl. Leave half the apples in the bowl. Place the other half in a sauté pan or saucepan and heat on medium until some of the juices cook out and the apples soften, about ten minutes. Remove the apples from the pan with a slotted spoon and stir with the uncooked apples to cool.
In a separate bowl, stir both the sugars with the cinnamon, allspice, and nutmeg to blend. Stir this into the apples and set aside.
On a lightly floured surface, roll out the first disc of dough into a circle just less than 0.5 centimeters thick. Dust a 23 centimeter pie plate with flour and line the bottom and sides of the plate with the pastry so that it hangs over the edge of the plate a little. Sprinkle the bottom of the shell with the oats (this will help absorb any excess juices). Spoon the apple filling into the shell and dot with the butter. Roll out the remaining disc of dough to 0.5 centimeters thick and place over the apples. Trim off any excess dough and pinch the edges of the crust together, creating a fluted edge. Brush the pastry with the egg wash. Stir a little sugar and cinnamon together and sprinkle generously over top. Use scissors to snip an opening in the top crust to allow steam to escape.
Place the pie on the prepared baking tray and bake for ten minutes. Reduce the oven temperature to 375°F (190°C) and continue to bake for another 40–50 minutes, until the crust is golden brown and the filling is bubbling. Cool the pie in the pan on a cooling rack for at least two hours before serving.