Preheat oven to 425˚F.
If you're using frozen pie crusts, allow them to thaw at room temperature for about 30 minutes. Remove one from its dish, and gently flatten it out on a lightly floured surface using a rolling pin. Set aside.
If you've made your own pie crust, roll out your second disk of dough into a 12" circle on a lightly floured surface with a rolling pin, and set aside.
In a large bowl, combine apples, sugar, flour, vinegar, cinnamon, salt, and nutmeg. Transfer the filling to the pie dish and use a spoon to spread evenly over the crust. Brush the edges of pastry with some egg, then top with the second pie crust. Trim the edges with a sharp knife and crimp with your fingers. Brush the top of the pie with egg and sprinkle with a little sugar. Make four slits in the top of the crust and poke holes all over it.
Place the pie in the oven and bake for 20 minutes. Lower the heat to 350˚F and continue to bake until the crust is golden brown and the filling is tender (use a knife through one of the slits to check) and is bubbling though the slits, about 40 minutes more. Transfer to a wire rack and let cool for at least 2 hours before serving.