· 6 cups (1 ¾ pounds) blackberries
· 1 cup sugar
· 3 tablespoons cornstarch
· 1 teaspoon lemon juice
· ½ teaspoon grated lemon zest
· 2 frozen deep dish pie shells
· 2 tablespoons unsalted butter, diced
· 1 large egg
Preheat the oven to 400 degrees F with a baking sheet on the middle rack. In a large bowl, toss the blackberries, sugar, cornstarch, lemon juice and lemon zest. Let stand, mixing and coarsely mashing occasionally, 20 minutes.
Cut a round hole in the center of one of the pie crusts using a small glass while still in the pie tin. Carefully remove the disc of dough and place the pie crust in the refrigerator until ready to use.
Spoon the mixture into the remaining pie shell. Dot the top with the butter. Place the first pie shell on top and separate the edges from the pie tin. Carefully remove the top pie tin and gently crimp the edges to the two shells together. Lightly beat the egg with 1 tablespoon water and brush on top and edge of the pie.
Place the pie on the hot baking sheet in the oven and bake until the filling is bubbling, 45 minutes to 1 hour. If the crust browns too quickly, cover it with foil and continue baking. Let the pie cool before serving.