Prepare the Crust: If you're making your own crust, be sure to chill the dough for at least 1 hour in the refrigerator before rolling. Let it sit at room temperature for 5–10 minutes before working with it. If using pre-made pie dough, follow the package directions.
Mix the Spiced Apple Filling: In a large bowl, combine the apple slices with the white sugar, brown sugar, flour, cinnamon, nutmeg, lemon juice, and lemon zest. Toss everything thoroughly so each slice is well coated in the sugary-spice mixture. Let this sit while you roll out your crust.
Assemble the Bottom Crust: Lightly flour your surface and roll one disc of pie dough into a 12-inch circle, about ⅛ inch thick. Gently transfer the dough to a deep-dish 9-inch pie plate, pressing it into the bottom and up the sides.
Fill the Pie: Spoon the apple filling evenly into the pie shell, leaving behind any extra juice in the bowl to avoid a soggy crust.
Top with Second Crust: Roll out the second pie dough to the same thickness and size. Drape it over the filling. Trim any excess dough hanging over the edges with a sharp knife. Seal the edges by folding them under and pressing them together gently, then crimp or flute as desired.
Vent and Finish the Crust: Cut 4 small slits into the top crust to allow steam to escape. Brush the top with the beaten egg, making sure to coat evenly. Sprinkle with sanding sugar for a sparkling finish, if using.
Protect and Bake: Cover the pie’s edge with a pie shield or strips of aluminum foil to prevent over-browning. Place the pie on a baking sheet and bake in a preheated 400°F (204°C) oven for 25 minutes.
Lower Heat and Continue Baking: Carefully remove the pie shield or foil. Reduce the oven temperature to 375°F (190°C) and continue baking for another 30–35 minutes, or until the crust is golden brown and the filling is bubbling through the slits.
Cool Before Serving: Remove from the oven and let the pie cool at room temperature for at least 3 hours to allow the filling to set properly before slicing.