- Make the crust: whisk the flour, salt and sugar. Cut in the butter until the largest pieces are the size of peas. Sprinkle in ice water a tablespoon at a time, tossing, until the dough holds together when squeezed.
- Divide into two disks, wrap and refrigerate at least 1 hour.
- Peel, core and slice the apples ¼ inch thick. Toss with the sugar, flour, lemon juice, cinnamon, nutmeg and salt.
- Heat the oven to 425°F (220°C) with a rack in the lower third. Roll one disk into a 12-inch circle and fit it into a 9-inch pie plate.
- Pile in the filling, mounding it in the center. Roll the second disk, lay it over the top, trim, fold the edges under and crimp. Cut 4 or 5 steam vents.
- Brush with the egg wash and sprinkle with coarse sugar. Chill 15 minutes.
- Bake 20 minutes. Lower the oven to 375°F (190°C) and bake 45 to 55 minutes more, until the crust is deep golden and juices bubble through the vents. Cover the edges with foil if they brown too fast.
- Cool on a rack for at least 3 hours before slicing.