Preheat oven to 350°F (175°C) and line a 9x13-inch baking pan with parchment paper.
In a bowl, mix flour, sugar, and salt, then cut in cold butter until crumbly.
Add egg and mix until dough forms.
Press two-thirds of dough into bottom of prepared pan.
In another bowl, toss apples with brown sugar, lemon juice, cinnamon, nutmeg, and cornstarch.
Spread apple mixture evenly over crust.
Crumble remaining dough over apple layer.
Bake 40–45 minutes until golden brown and filling bubbles.
Cool completely before slicing into bars.