In a large bowl, whisk together the all-purpose flour, granulated sugar, and table salt. Add the diced, very cold unsalted butter to the bowl and toss it with the flour mixture to coat. Using your fingers (or a pastry blender or two knives), work the butter into the flour until the mixture is crumbly and the butter pieces are just larger than pea-sized. Pour in 1/2 cup of ice water and use a rubber spatula to mix until the dough starts to come together. If the dough is too dry, add more water, 1 tablespoon at a time, up to 1/4 cup extra, just until the dough comes together. Bring the dough together with your hands, forming a soft, slightly loose ball. Divide the dough into two pieces, one a bit larger for the bottom crust. Wrap each piece tightly in plastic wrap and shape them into 1-inch thick rectangles. Refrigerate the dough for at least 2 hours to chill thoroughly before using. I prefer using my fingers to rub the butter in, as it gives me the best control over the texture.

In a very large bowl, toss the peeled and chopped apples with lemon juice to prevent browning and add a subtle brightness. In a separate medium bowl, mix together the sugar, cornstarch, ground cinnamon, ground nutmeg, ground allspice, and table salt. Sprinkle this sugar-spice mixture over the apples and toss well until all the apple pieces are thoroughly coated. Set the prepared apple filling aside while you roll out the dough. I like to taste my apples before mixing in the sugar and adjust the amount based on their sweetness.

Preheat your oven to 375°F. Line the bottom of a 10×15×1-inch baking sheet (or similar-sized jelly roll pan) with parchment paper. On a well-floured surface, roll out the larger piece of chilled dough (from Step 1) into an 18×13-inch rectangle. Transfer this dough to the lined pan, draping the edges over the sides and gently pressing it into the corners. Place the pan in the fridge or freezer while you roll out the second, smaller piece of dough into a 16×11-inch rectangle. Remove the pan from the fridge/freezer and evenly spread the prepared apple filling (from Step 2), including any juices, over the bottom crust. Lay the second rolled dough over the apples, and trim both the top and bottom crusts so they hang about 3/4-inch beyond the edge of the pan. Press the edges together to seal tightly with your fingers.

Brush the top crust of the assembled slab pie with either the heavy cream or egg wash to encourage a golden, glossy finish. Using a sharp knife, cut several 1-inch slits in the top crust to allow steam to escape during baking. Set the jelly roll pan on a larger baking sheet to catch any possible spillage. Bake in the preheated oven for 40 to 45 minutes, or until the top crust is a deep golden brown and the apple filling is bubbling through the slits. For a more rustic look, I like to sprinkle a little extra sugar over the top right before baking.

Once baked, transfer the pan to a wire rack and let the pie cool for at least 45 minutes before cutting and serving. This resting period allows the filling to set, making for neater slices and better flavor. I always recommend exercising patience here—it really makes the difference between a runny and a picture-perfect pie.