Apple Crumble Slab Pie

To make the crust: Whisk together the flour, salt, and buttermilk powder., Add the shortening, working it in until evenly incorporated., Work in the butter until the mixture is unevenly crumbly; you want big chunks of butter in among the smaller ones., Add 6 tablespoons water, and toss to combine. Toss with enough additional water to make a chunky, fairly cohesive mixture. It should hold together when you gather it up and squeeze it in your hand., Form the dough into a rectangle, wrap it in plastic, and refrigerate for 1 hour., Place the dough on a large floured work surface. Roll it into a 15" x 20" rectangle; the dough will be thin, only about 1/8" thick., Starting with a short end, roll the crust around your rolling pin and transfer it to an ungreased or parchment-lined 13" x 18" (half sheet) pan, pressing it into the corners and edges and using any overhanging scraps to patch any cracks or uneven edges., Refrigerate the crust while you prepare the filling and crumble., Preheat the oven to 375°F., To make the filling: Combine the sliced apples and lemon juice in a large mixing bowl., In a small bowl, whisk together the Instant Clearjel or flour, the sugar, spice, and salt. Sprinkle the mixture over the apples, stirring to coat. Stir in the boiled cider and melted butter. Set the mixture aside., To make the crumble: Stir together the oats, sugar, flour, spice, and salt. Work in the butter until the mixture is unevenly crumbly. Set it aside., Spoon the filling into the chilled crust, then top with the crumble., Bake the pie for 55 to 65 minutes, until the crust and crumble are golden and the filling is bubbling., Transfer the pie to a rack to cool; it will hold together better if you let it cool completely before serving. (Though honestly, who'll really take offense if you serve them a warm, slightly messy piece of pie?)