Make filling. Place berries in medium saucepan, with the lemon juice, sugar, and salt and set over medium heat.
Cook down berries. Cook over medium heat, and stirring frequently most of the berries have been broken down and mixture is thickened about 8-10 minutes.
Add cornstarch. Stir in cornstarch and continue to cook for 1-2 minutes more to thicken. Remove from the heat, stir in the butter, and allow to cool before spooning into pie crust.
Prep oven. Adjust oven rack to lowest position, place a rimmed baking sheet (I also like to line mine with foil for easy clean-up) on oven rack and heat oven to 400°F. I like to pre-heat the baking sheet while I prepare the pie so it is nice and warmed and helps brown the bottom crust.
Roll out pie crust. Remove one ball of dough from the fridge, and on a lightly floured counter roll out the dough into a 12-inch circle, turning the dough after a few rolls to ensure the dough isn’t sticking to your surface.
Transfer pie crust to pie plate. Fold the dough in quarters, and then transfer to your 9” pie plate. Carefully unfold the dough, and tuck in with your fingers.
Add filling. Spoon berry pie filling into the pie crust.
Make lattice top. Remove the second disc of dough and roll out into a 12” circle. Cut the circle into strips, about 1-2” wide. Place strips vertically across pie (about 6). Then fold every other strip back and place another strip going horizontally across. Use the longer strips in the middle of the pie, and the shorter strips on the ends. Press the ends of the strips into the bottom crust. Trim any excess dough. Crimp with a fork to seal.
Egg wash. In a small bowl, stir together the egg and milk. Brush the crust with egg wash. Sprinkle top of crust with coarse sugar, if using.
Bake pie. Place pie on large baking sheet (to catch drips) and bake for 30 minutes at 400°F. Reduce oven temperature to 350°F and continue to bake until juice bubbles (it's important you see bubbling to ensure the cornstarch has fully thickened) and crust is deep golden brown, 30 to 40 minutes longer. If pie crust edges is browning too much, cover with tinfoil or pie shield. Transfer pie to wire rack; cool to room temperature, at least 4 hours. Cut into wedges and serve.