In a mixing bowl, combine all-purpose flour, salt, and sugar. Cut in chilled, cubed unsalted butter until resembling coarse crumbs. Gradually add cold water, mixing until the dough forms. Wrap in plastic wrap and refrigerate for 30 minutes.

Prepare the apple filling by peeling, coring, and dicing apples. Combine diced apples with brown sugar, cinnamon, lemon juice, and cornstarch. Stir until evenly coated and set aside.

Preheat your oven to 375°F (190°C). Roll out chilled dough into a rectangle, about 12x16 inches. Transfer to a prepared baking sheet.

Spoon apple filling down the center of the dough, leaving about an inch on each side. Cut diagonal slits on both sides and fold over the filling, alternating sides.

Bake for 25-30 minutes until golden brown and flaky. Let cool for 10 minutes before glazing.

Whisk powdered sugar, milk, and vanilla extract in a small bowl until smooth. Adjust milk for desired glaze consistency. Drizzle over cooled pastry.

Slice into pieces and serve warm at room temperature.