Preheat oven to 400 degrees F. Line a sheet pan with parchment paper.

On a lightly floured surface, roll one disk of pie crust out into a rough 13-inch circle. Transfer to a 9 or 10-inch pie dish. Roll the second disk or pie crust out and have it ready.

In a large bowl, combine the sugar, cornstarch, and lemon zest. Add the blueberries and blackberries and gently mix until they are coated with the sugar mixture.

Dump berry mixture into crust-lined pie dish. Place the other pie crust on top, or form a lattice, and press the bottom and top crusts together to seal. Tuck the ends into the pie dish and crimp crust edges as desired. With a sharp knife, cut slits into the top of the pie for vents (if making a lattice pie, skip this step).

Brush with egg wash and sprinkle with coarse sugar.

Place pie on the prepared sheet pan and bake until pie crust is golden and juices are visibly bubbling through the vents or lattice, about 1 hour. If top crust browns too quickly, cover with foil.

Let cool completely before serving, for best results. If desired, you can reheat the pie in a 350 degree oven for 30 minutes. Serve with whipped cream or vanilla ice cream, if desired.