Cut apples into wedges, 1/2" thick at the widest point. Mix sugar, nutmeg, cinnamon, and salt together in a large bowl. Add apples and toss well.
Preheat oven to 425°.
Roll out 1st sheet of refrigerated pie dough and fit into a 9" pie pan. Fill the pastry with apples and all their juices, and dot with butter. Roll out remaining dough into a 12" round, place on top of apples, then fold edge under and crimp or flute. Poke 4 or 5 holes in top crust with a knife to allow steam to escape during baking.
Beat 1 tbsp. water with egg then brush top of pie all over. Bake until crust is golden brown, about 45 minutes. Allow to cool slightly before serving.
Tip: If the edges of your crust are getting too dark cover just the edges with a ring made out of foil.