2 disks refrigerated pie dough (one 14-ounce package)
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.
Fit one round of dough into a 9-inch pie plate. Trim the edges, leaving a 1/2-inch overhang. Refrigerate while you continue. Cut the remaining piece of dough into approximately eleven 11/4-inch-wide strips.
Spoon the blackberry mixture into the pie shell. Dot the top with the butter. Arrange the dough strips in a lattice pattern on top of the filling and trim the strips close to the edge of the pan; roll up and crimp the edge. Lightly beat the egg with 1 tablespoon water and brush on the 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.
Find a lattice how-to, or just lay half of the dough strips evenly on the pie and arrange the remaining strips diagonally across them.