Preheat the oven to 425°.
Line the pie dish with a 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 the edges, and cut away excess dough.
Make several small slits in the top to allow steam to escape.
Mix together egg yolk and water and brush over pie.
Cover the pie loosely with aluminum foil and place on a baking sheet.
Bake for 45 minutes, covering with foil after 20 minutes.
Cool on a wire rack.