Whisk the pastry flour, sugar and salt in a bowl. Work in the cold butter with a pastry blender or fingertips until a mixture of small flakes and pea-size pieces remains. Add 6 tablespoons ice water, tossing gently; add more a tablespoon at a time only until the dough holds together when squeezed.

Gather into two discs without kneading, making one slightly larger. Wrap and refrigerate for at least 60 minutes. Allow a very firm disc a few minutes at room temperature before rolling.

Set a rack in the lower third and heat the oven to 425°F (220°C). Toss the apple slices with lemon juice. Mix the filling sugar, flour, cinnamon, nutmeg and salt, then toss thoroughly with the apples.

Roll the larger dough disc on a lightly floured surface to about 12 inches (30 cm). Ease it into a 9-inch metal pie pan without stretching. Arrange the apples compactly, mounding the center modestly. Add any juices remaining in the bowl and dot with the filling butter.

Roll the second disc to about 11 inches (28 cm). Lay it over the apples. Trim excess pastry to about 1 inch beyond the rim, fold the edges together underneath, and crimp. Cut five steam vents near the center. Mix the measured egg and water and brush a thin layer over the top. If the pastry has warmed and softened, chill the assembled pie for 15 minutes before baking.

Place on a rimmed baking sheet. Bake for 20 minutes, then reduce the oven to 375°F (190°C) and bake about 50–60 minutes more. Shield the rim or tent the top with foil if browning too quickly. The crust should be deeply golden, thick filling juices should bubble through the central vents, and a thin knife through a vent should meet tender apples. Extend the baking time if needed.

Transfer the pie to a rack and cool at least 4 hours before slicing. The filling thickens as it cools. Cut into 8 slices.