Prepare the Dough:
In a large bowl, mix together the flour, sugar, and salt. Add the cold butter pieces and toss to coat them with flour.
Using your hands, a pastry blender, or two knives, break the butter into small pieces, slightly larger than peas.
Gradually add 1/2 cup of cold water and stir with a rubber spatula until the dough begins to come together. If it’s still dry, add a little more water, one tablespoon at a time, until the dough forms a soft, slightly loose ball.
Divide the dough into two portions—one slightly larger than the other. The larger portion will be for the bottom crust. Shape each portion into a 1-inch thick rectangle. Wrap each piece in plastic wrap and refrigerate for at least 2 hours.
Prepare the Filling:
In a large bowl, toss the apple pieces with lemon juice.
In a medium bowl, combine the sugar, cornstarch, cinnamon, nutmeg, allspice, and salt. Sprinkle this mixture over the apples and stir to coat them evenly. Set aside.
Assemble the Slab Pie:
