To Make the Crust:
Measure the flour into a large mixing bowl. Whisk in the sugar and salt.
Toss the cold butter cubes into the flour mixture, ensuring they are well coated. Using your fingers, mash the butter into smaller pieces, just bigger than the size of peas (or use a pastry blender or two knives).
Stir in ½ cup of cold water with a rubber spatula until the water is absorbed and the dough starts to come together. Add the remaining ¼ cup of water, 1 tablespoon at a time, if needed, until the dough just comes together.
Divide the dough into two pieces, one slightly larger than the other (the larger one will be used for the bottom crust). Shape each piece into a 1-inch thick rectangle, wrap in plastic wrap, and refrigerate for at least 2 hours.
To Make the Filling:
In a large bowl, toss the apple chunks with a squeeze of lemon juice.
In a separate medium bowl, combine the sugar, cornstarch, cinnamon, nutmeg, allspice, and salt. Sprinkle the sugar-spice mixture over the apples and toss well to coat. Set aside.
To Assemble the Apple Slab Pie:
Preheat the oven to 375°F (190°C). Line the bottom of a 10×15×1-inch baking sheet or jelly roll pan with parchment paper.
On a well-floured surface, roll out the larger piece of dough into an 18×13-inch rectangle. Transfer the dough to the prepared pan, draping the edges over the sides. Gently press the dough into the corners of the pan. Chill the dough in the fridge or freezer while rolling out the second piece of dough into a 16×11-inch rectangle.
Pour the apple filling, along with any accumulated juices, into the crust-lined pan. Spread it evenly. Drape the second sheet of dough over the apples.
Trim the dough edges so they hang about ¾-inch over the edge of the pan. Seal the edges tightly with your fingers.
Brush the top of the dough with heavy cream or egg wash. Using a sharp knife, cut several 1-inch slits in the top to allow steam to escape.
Place the pan on a larger baking sheet to catch any potential spillage during baking.
Bake for 40 to 45 minutes, or until the crust is golden brown and the filling is bubbling through the slits.
Let the pie cool on a wire rack for at least 45 minutes before cutting and serving.