Assembly

• Combine ½ cup sugar, cornstarch, salt, and lemon zest in a large saucepan.
• Add the blackberries and lemon juice and toss to coat.
• Lightly smash the blackberries with the back of a spoon to release some of their juices.
• Cook over medium heat, stirring often, until the juices bubble and thicken, about 7 minutes. Taste and add the rest of the sugar, if desired.
• Chill the blackberry filling for at least 1 hour before using, up to overnight.
• On a floured surface, roll out one disc of pie dough into a 13-inch round. Place it in the bottom of a 9-inch pie dish.
• Pour the blackberry filling into the dish.
• Roll the second disc of dough into a 12-inch circle. Either cut into strips for a lattice top, or place the entire round on top of the filling, crimp edges and cut vents.
• Chill the pie for 1 hour.
• Heat the oven to 375°F.
• Brush the top of the pastry with the beaten egg.
• Bake for 45-50 minutes, until the crust is golden brown and the filling bubbles up.