tablespoon milk (or water, lemon juice or fresh apple cider, plus a drop or two more if needed)
Make pie crust: Whisk together flour, sugar and salt in the bottom of a large bowl. Using a pastry blender, 2 forks, or your fingertips, work the butter into the flour until the biggest pieces of butter are the size of peas. (You'll want to chop your butter into the small bits first, unless you're using a very strong pastry blender in which case you can throw the sticks in whole.).
Gently stir in the water with a rubber spatula, mixing it until a craggy mass forms. Get your hands in the bowl and knead it just two or three times to form a ball. Divide dough roughly in half, one being slightly larger than the other. Wrap each half in plastic wrap and flatten a bit, like a disc.
Chill in fridge for at least an hour or up to two days or slip plastic wrapped dough into a freezer bag and freeze for up to 1 to 2 months. To defrost, leave in frig for 1 day.
Heat oven to 375 degrees. Line bottom of 10x15x1 inch baking sheet or jellyroll pan with parchment paper.
Prepare filling: In a large bowl, toss apples with lemon juice until coated. Top with remaining filling ingredients and stir to evenly coat.
Assemble pie: On a lightly floured surface, roll one of your dough halves (the larger one first) into an 18x13 inch rectangle. Do your best to work quickly, keeping the dough as cold as possible and using enough flour that it doesn't stick to the counter. Transfer to your prepared baking sheet and gently drape some of the overhang in so that the dough fills out the inner edges and corners. Some pastry will spill over the sides of the pan; trim this to 3/4 inch.
Pour apple mixture over prepared pan and spread evenly.
Roll the second of your dough halves (the smaller one) into a 16x11 inch rectangle. Drape over filling and fold the bottom crust's overhang over the edges sealing them together. Cut small slits to act as vents all over the lid. Brush lid with heavy cream or egg wash. Bake until crust is golden and filling is bubbling, 40-45 minutes. Transfer to a wire rack until just warm to the touch, about 45 minutes.
In a medium bowl, stir together confectioner's sugar and liquid of your choice until a pourable glaze consistency is reached. Use a spoon to drizzle over top. Serve slab pie in squares or rectangles, warm or at room temperature.