Preheat oven to 400˚F.
In a large bowl, whisk together the sugar, cornstarch, cinnamon, and salt.
In a larger bowl, toss the apples with the lemon juice. Add the sugar mixture and toss again.
Pour the apples into a pie plate lined with 1 flaky pie dough. Dot with butter.
Roll out the other smaller piece of dough and place it over the apple pie filling. Turn the edges of the top layer of pie dough under the bottom layer of pie dough, working all around the edges of the pie. Crimp the edges of the pie dough with your fingers as you work around the pie.
Whisk the egg and water together. Using a pastry brush, brush the pie crust with the egg wash. Sprinkle the top with the sugar.
Make 4 to 5 2~inch slits in the top dough, slightly opening each one.
Place the pie on a sheet pan lined with aluminum foil.
Bake the pie in the preheated oven until the crust is golden brown and the juices begin to bubble through the slits, about 45 ~ 60 minutes. Rotate the pie halfway through the baking time. Start checking the doneness at 45 minutes.
The pie edges may be covered with a pie shield or a 3" strip of aluminum foil to prevent excessive browning.