Mix flour, salt, and sugar. Cut in cold butter until pea-sized pieces form. Add ice water one tablespoon at a time until the dough just holds together. Divide into two discs, wrap in plastic, and chill for at least 1 hour.

Peel, core, and slice apples to ¼-inch thickness. Toss with sugar, cornstarch, cinnamon, nutmeg, and lemon juice. Rest for 20 minutes, then pour off and discard all accumulated liquid.

Preheat oven to 425°F (220°C). Roll one dough disc into a 12-inch circle and transfer to a 9-inch pie dish. Add the drained filling in an even layer. Dot with butter pieces.

Roll the second dough disc and lay over the filling. Fold the top crust under the bottom crust edge and crimp firmly to seal. Cut 6 to 8 vents in the top. Brush with egg wash and sprinkle with coarse sugar.

Place on a rimmed baking sheet. Bake at 425°F for 20 minutes. Reduce heat to 375°F (190°C) and bake 30 to 40 minutes more until deep golden and filling is bubbling through the vents.

Cool on a wire rack for at least 2 hours before slicing. Do not rush this step.