Preheat oven to 325 degrees F (165 degrees C). Line a pie dish with 1 crust.
Whisk together cornstarch, sugar, cinnamon, nutmeg, and salt in a large bowl. Stir apples, maple syrup and whole egg into cornstarch mixture.
Pour apple mixture into the prepared crust. Cover with top crust, seal edges, and cut away excess dough. Make several small slits in the top to allow steam to escape. Whisk together egg yolk and water in a small bowl; brush over the pie. Cover pie loosely with aluminum foil and place on a baking sheet.
Bake in the preheated oven for 35 minutes. Remove foil; continue baking until crust is golden brown, about 15 minutes. Cool on a wire rack.
