In a large mixing bowl, whisk together flour, sugar, and salt until well combined. Cut in cold butter until resembling coarse crumbs.

Add the egg, ice water, and lemon juice. Mix gently until a shaggy dough forms. Divide into two pieces, flatten into discs, wrap in plastic wrap, and refrigerate for at least 30 minutes.

Peel, core, and slice Granny Smith apples. Toss with sugar, cornstarch, cinnamon, nutmeg, lemon juice, and vanilla extract.

Preheat your oven to 375°F (190°C). Roll out the larger piece of dough to fit a greased 9x13-inch baking pan. Transfer to the pan.

Spread the apple filling evenly over the crust. Roll out the second piece of dough, cover the filling, crimp edges, and cut slits in the top.

Bake for 45-50 minutes or until golden brown and the filling is bubbling.

Whisk together powdered sugar, melted butter, and enough milk for a smooth glaze.

Drizzle glaze over the cooled slices, slice into squares, and serve.