1. On a lightly floured piece of parchment, roll one disk of dough into a 12-by-17-inch rectangle. Transfer crust to a 10 1/2-by-15 1/4-inch rimmed baking sheet or jelly-roll pan and press into bottom edges and up sides. Fold overhang under and around edge. Refrigerate 30 minutes.

2. In a small bowl, combine room-temperature butter, brown sugar, cinnamon, and 1/4 teaspoon salt. On a lightly floured piece of parchment, roll second disk of dough into a 12-by-16-inch rectangle. Evenly spread butter mixture over dough all the way to ends, then, starting at a long edge, tightly roll into a log. Transfer to a baking sheet and refrigerate until firm, about 30 minutes.

3. Preheat oven to 425 degrees with a rack in lower third and a foil-lined baking sheet on a rack below. In a large bowl, toss together apples, lemon juice, granulated sugar, flour, remaining 1/4 teaspoon salt, and vanilla. Fill piecrust with apple mixture and dot with cold butter.

4. Cut dough log into 1/4-inch slices, rotating with each slice to keep slices round. (You'll need 63 slices total.) Arrange slices over apple mixture, slightly overlapping, in nine rows of seven. Refrigerate until firm, about 30 minutes.

5. Brush top of pie with egg wash and sprinkle with sanding sugar. Bake until top crust is just set, 20 minutes. Reduce temperature to 375 degrees and continue baking until crust is golden brown and juices are bubbling, about 1 hour. Let cool completely on a wire rack. Slab pie tastes best the day it's made.