5 cups all-purpose flour (spooned and leveled), plus more for work surface
1 teaspoon fine salt
3/4 cup plus 2 tablespoons sugar, divided
2 cups (4 sticks) cold unsalted butter, cut into small pieces
2/3 cup ice water (plus up to 1/2 cup more for pulsing)
3 3/4 pounds peaches, thinly sliced (12 cups)
1/4 cup cornstarch
1 tablespoon finely grated lemon zest plus 2 tablespoons lemon juice
In a food processor, pulse flour, salt, and 2 tablespoons sugar until combined. Add butter.
Pulse until mixture resembles coarse meal, with a few pea-size pieces of butter remaining. Then, with machine running, add 2/3 cup ice water.
Pulse until dough is crumbly but holds together when squeezed (if necessary, add up to 1/2 cup ice water, 1 tablespoon at a time). Do not overmix.
Divide dough into 2 disks and wrap each tightly in plastic. Refrigerate until firm, at least 1 hour (or up to overnight).
Preheat oven to 400 degrees. In a large bowl, toss together peaches, 3/4 cup sugar, cornstarch, lemon zest and lemon juice.
Lightly flour a work surface and rolling pin. Roll out 1 disk dough to a 12-by-16-inch rectangle. Roll dough over rolling pin; carefully unroll into a 10-by-14-by-1-inch rimmed jelly-roll pan. Press gently to fit into pan.
Pour in peach filling, then lightly brush edges of dough with water. On floured surface, roll out remaining disk to an 11-by-15-inch rectangle. Roll dough over rolling pin and carefully unroll over filling.
Press along moistened edges to seal. Fold under any overhang, tucking it into the pan. With your fingers, crimp edges.
With a paring knife, cut slits on top to vent. Place pie in oven, then reduce heat to 375 degrees. Bake until crust is golden and juices are bubbling, 45 to 50 minutes. Let cool on a wire rack 1 hour. Serve warm or at room temperature.