Open one package of pie crust and stack two of the crusts on top of each other. On a lightly floured surface, roll the two crusts together into a rectangle that’s roughly 18x12 inches.
Press the crust into a 15x10x1 inch pan, allowing the edges to overhang.
Peel and dice the apples.
In a large bowl, stir together the apples, sugar, cornstarch, cinnamon, nutmeg, and lemon juice.
Spread the mixture evenly over the top of the crust.
Stack the two remaining crusts together and roll out into a second 18x12 inch rectangle.
Place the second pie crust on top of the apples and pinch the edges together. Use a knife to cut slits along the top of the crust to vent.
Whisk the egg until smooth and brush over the top of the crust. Sprinkle sugar over the top of the egg.
Notes
Storage: Keep leftover apple slab pie covered tightly in the pan or put slices in an airtight container. Store at room temperature for up to 2 days or in the fridge for up to 5 days.