Make the crust: In a bowl, mix flour, sugar, and salt. Cut in butter until crumbly. Add ice water one tbsp at a time just until dough holds together. Flatten into a disk, wrap, and chill 30+ minutes.
Prep filling: Toss apple slices with both sugars, cinnamon, lemon juice, and flour. Let sit 10 minutes.
Roll out crust: On a floured sheet of parchment, roll dough into a 12-inch circle. Transfer parchment and dough to a baking sheet.
Assemble: Arrange apples in the center, leaving a 2-inch border. Fold edges over, pleating as you go.
Brush & sprinkle: Brush crust with egg wash and sprinkle with coarse sugar.
Bake: At 400°F for 35–40 minutes or until crust is golden and apples are tender.
Cool slightly: Best served warm with a scoop of vanilla ice cream!