Preheat your oven to 425°F (220°C).
On a floured surface, roll out one of the dough discs into a circle that's about 12 inches in diameter. Carefully transfer it to a 9-inch pie dish.
Pour the apple filling into the crust, spreading it out evenly.
Roll out the second dough disc. You can either place it on top to make a full top crust, or you can cut it into strips to make a lattice top.
If using a full top crust, cut several slits in it to allow steam to escape. If making a lattice, weave the strips over and under each other on top of the filling.
Beat the egg and water together for the egg wash. Brush it lightly over the top crust.
Bake for 45-55 minutes, or until the crust is golden and the filling is bubbling.
Remove from the oven and let it cool for at least 2 hours before serving.