Step 1

In a large bowl, combine the blackberries, granulated sugar, lemon zest, and lemon juice. Cover and let sit at room temperature for 1 hour so the juices are drawn out and the berries soften, stirring occasionally.

Step 2

Position an oven rack in the lower third of the oven and preheat to 425°F. Place a foil-lined rimmed baking sheet on the rack to preheat.

Step 3

On a floured surface, roll out 1 pie crust into a 12-inch round, starting at the center and working your way out. (Sprinkle flour over the top of the dough if it's a bit sticky.) Carefully lift the dough and place it in a 9-inch pie dish (not a deep dish); gently press the dough against the sides of the dish. Trim the overhanging dough to 1 inch at the edge of the pie dish. Cover and refrigerate until ready to use.

Step 4

On a floured surface, roll out the second pie crust into a 10-inch round. Place the dough on a baking sheet or large plate, cover, and refrigerate until ready to use.

Step 5

Drain the blackberries, reserving about ½ cup of the liquid. Discard any remaining liquid. Return the drained blackberries to the large bowl.

Step 6

In a medium saucepan, combine the cornstarch and salt. Gradually whisk in the reserved blackberry juice. Bring to a boil over medium heat, stirring constantly to prevent lumps. Use a rubber spatula to scrape the bottom and sides of the saucepan. Reduce the heat to medium-low, and cook until thickened, 2 to 3 minutes. Reduce the heat to low and whisk in the butter. When the mixture is well combined and glossy, remove from the heat. Pour the mixture over the blackberries, folding to combine. Scoop the mixture into the pie crust set in the pie dish.

Step 7

Place the remaining pie crust over the top of the berries. Trim the excess dough around the edges, and crimp the top and bottom crusts together to seal them. Cut a few vent holes in the top crust. Brush the top crust with the beaten egg and sprinkle with the turbinado sugar.

Step 8

Bake the pie on the preheated baking sheet until the filling is bubbly and the crust is deeply golden, 50 minutes to 1 hour, rotating the pie halfway through baking. (Cover the pie with foil if the crust starts getting too dark after 40 minutes.)

Step 9

Allow the pie to cool to room temperature on a wire rack before slicing, about 4 hours.