Mix flour, salt, and sugar in a bowl. Cut in cold butter until crumbly. Add ice water gradually until dough forms.

Divide dough into two portions, wrap, and refrigerate for 30 minutes.

Preheat oven to 375°F (190°C) and grease or line a 10×15-inch sheet pan.

Combine apples, granulated sugar, brown sugar, flour, cinnamon, nutmeg, lemon juice, and vanilla in a bowl and toss well.

Roll out the first dough portion into a rectangle and press into the sheet pan.

Spread apple mixture evenly over the crust.

Roll out the second dough portion and place over apples. Seal edges and cut slits on top.

Brush with egg wash and sprinkle coarse sugar if desired.

Bake for 40–45 minutes until crust is golden and filling is bubbling.

Allow pie to cool before slicing into squares and serving.