Preheat your oven to 425°F (220°C).
Prepare a 9-inch pie pan. Lightly grease with butter or nonstick spray.
To make the filling: In a large bowl, combine the apple slices, granulated sugar, cinnamon, nutmeg, salt, and tapioca flour until thoroughly mixed.
In a small bowl, Combine the granulated sugar, all-purpose flour, and cinnamon. Set aside.
To assemble the pie: On a floured surface, roll out one disc of chilled pie dough until it forms a 12" circle. Place it into a 9-inch pie dish.Spread the sugar, flour, cinnamon on the bottom of the crust.
Spread the sugar, flour, cinnamon on the bottom of the crust.
Spoon the prepared apple filling into the pie crust, piling it high and tightly.
Roll the second dough disc into a 12-inch circle. Place over apples. Trim excess, fold edges under, and crimp to seal.
Brush with egg wash and sprinkle with coarse sugar, if desired Cut 4 slits in the top crust.
Place pie on a parchment-lined baking sheet. Bake at 425°F (220°C) for 15 minutes, then reduce to 375°F (190°C) and bake 40–45 minutes, until golden and bubbling.
Cool at least 4 hours (or overnight) before slicing to allow the filling to set.
Store any leftover pie, lightly covered, at room temperature for several days. Freeze for longer storage.