Preheat the oven: Set your oven to 375°F (190°C).
Prepare the apples: Peel, core, and slice the Granny Smith apples into ½ inch slices. Place them in a large mixing bowl.
Make the filling: Add granulated sugar, light brown sugar, flour, cinnamon, nutmeg, lemon juice, and lemon zest to the apples. Mix well to coat the apples evenly.
Assemble the pie: Place one pie crust into a 9-inch pie dish. Pour the apple filling into the crust, spreading it evenly.
Top the pie: Cover the apple filling with the second pie crust. Trim any excess dough, then crimp the edges to seal. Cut small slits in the top crust to allow steam to escape.
Apply the egg wash: Lightly brush the top crust with the beaten egg. Sprinkle with sanding sugar if desired.
Bake the pie: Place the pie on the center rack of the oven and bake for 50-60 minutes, or until the crust is golden brown and the filling is bubbling.
Cool the pie: Allow the pie to cool for at least 2 hours before slicing. This helps the filling set properly.
