2. In a large bowl, mix together the apples, blackberries, sugar, cornstarch, lemon juice, and Calvados or vanilla extract.
3. Preheat the oven to 375ºF (190ºC). Have a rimmed baking sheet ready that’s approximately 18 x 13-inches (46 x 33cm).
4. 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.
5. 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.
6. 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.)
7. Let the pie cool a little before slicing.