In a large bowl, mix together the apples, blackberries, sugar, cornstarch, lemon juice, and Somerset Ridge Avalon
Preheat oven to 375 degrees
Have a rimmed half sheet pan (18x13 inches) ready
On 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 tot he baking sheet; it may help folding it in half before lifting the dough, then unfolding it once - you've got it into the pan
Spread the apple and blackberry mixture over the dough
Roll the remaining rectangle of dough so it's also a littler larger than the baking sheet
Drape the dough over the apples
Fold the excess overhang of dough around the edges of the pan underneath the pie
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
Brush the egg wash over the top of the pie
Sprinkle liberally with the granulated sugar
Bake pie until it's deep golden brown on top and thick juices are bubbling from the slits on top (about 45 minutes)
Let the pie cool about 15 minutes before slicing