Apple Blackberry Slab Pie

You'll need about 6 pounds (2,75kg) to end up with the right amount of apples. You can use fresh or frozen blackberries. If using frozen, don't defrost before using. In the winter months, especially if you want to make this holiday-friendly, feel free to replace the blackberries with a few handfuls of fresh or frozen cranberries, or scatter bits of traditional mincemeat or quick mincemeat over the apple filling before covering the pie with the top layer of dough. I don't use too much sugar in the pie but if you have very tart apples, or like a sweeter filling, you can use 1 cup (200g) of sugar. I like to sprinkle the top generously with granulated (free-flowing) natural sugar, which goes by several names, but it's darker in color than white sugar and the large crystals give the pie a supremely satisfying crunch. Turbinado or demerara sugar fall into that category; in France, it's called cassonade. French people don't habitually add cinnamon to apple desserts, but if you'd like to do it, you're welcome to add a dusting of it to the apple and blackberry filling mixture.

Servings servings

For the dough

4 cups (560g) flour
2 tablespoons granulated sugar
1 1/2 teaspoons salt
1 teaspoon baking powder, preferably aluminum-free
1 1/2 cups (12 ounces, 340g) unsalted butter, cubed and chilled
2/3 cup (160ml) ice water, plus more if necessary

For the filling

12 cups (3 pounds, 1,3kg) peeled, sliced or cubed apples
2-3 cups (8-12 ounces, 225-340g) blackberries
2/3 cup (170g) granulated sugar
2 tablespoons cornstarch
4 teaspoons freshly squeezed lemon juice
2 tablespoons Calvados, Cognac, or brandy, or 2 teaspoons vanilla extract

For finishing the pie

1 egg
1 teaspoon milk
3-4 tablespoons granulated natural sugar (see headnote), or granulated sugar

To make the dough

In a stand mixer fitted with the paddle attachment (or by hand in the large bowl using a pastry blender) mix the flour, sugar, salt, and baking powder. Add the cubed butter and mix on medium speed until the butter is broken up into pieces the size of corn kernels. Add the water and continue to mix until the dough just starts to come together. Use your hands to gather the dough into one cohesive mass. (If it feels dry, you can add more ice water, 1 tablespoon at a time, until it's wet enough to hold together, but don't overdo it.) Divide the dough in two and shape each piece of dough into a rough rectangle about 1-inch (3cm) high. Wrap each piece of dough in plastic wrap and chill for a few hours, or overnight.

To make the filling and assemble the pie

In a large bowl, mix together the apples, blackberries, sugar, cornstarch, lemon juice, and Calvados or vanilla extract.
Preheat the oven to 375ºF (190ºC). Have a rimmed baking sheet ready that's approximately 18 x 13-inches (46 x 33cm).
On a lightly floured surface, roll one rectangle of the dough so that it's a little larger than the dimensions of the baking sheet. Transfer the dough to the baking sheet; it may help folding it in half before lifting the dough, then unfolding it once you've got it in the pan. Spread the apple and blackberry mixture over the dough.
Roll the remaining rectangle of dough so it's also a little larger than the baking sheet, then drape the dough over the apples. Fold the excess overhang of dough around the edges of the pan underneath the pie then crimp the edges. Make 8 slits in the top of the pie with the tip of a paring knife.
Mix the egg and milk together then brush the egg wash over the top of the pie. Sprinkle liberally with the granulated natural sugar then bake the pie until it's deep golden brown on top and thick juices are bubbling from the slits in the top of the pie, about 45 minutes.(Note: I didn't have this issue but you may want to line the oven rack you'll be baking on with foil, or the rack underneath, before baking the pie, to catch any sticky juices that may bubble over.)
Let the pie cool a little before slicing.

Notes

Serving: Serve the pie warm or at room temperature with a favorite ice cream, such as Vanilla Ice Cream or Cinnamon Ice Cream.

Storage: The dough can be made up to three days in advance and refrigerated. It can also be frozen for up to two months. The pie will keep for up to three days at room temperature but the dough will become less-crisp after the first day.