Preheat oven to 375°F. Unwrap the larger Slab Pastry Crust disk; roll dough out on a lightly floured work surface into an 18- x 13-inch rectangle. Wrap dough around rolling pin, and gently fit inside a 15- x 10-inch jelly-roll pan. Turn dough edges under; crimp. Prick bottom and sides of dough with a fork. Line with parchment paper, leaving a 3- to 4-inch overhang around edges of pan. Fill with pie weights or dried beans. Bake 10 minutes. Remove parchment and pie weights. Return crust to oven, and bake at 375°F for 5 minutes. Transfer to a wire rack, and cool completely, about 30 minutes.

Place peaches, cherries, nectarines, and vanilla in a large bowl. Stir together granulated sugar, brown sugar, flour, and salt in a medium bowl. Sprinkle over peach mixture; stir gently to combine. Spoon mixture into prepared crust, and drizzle evenly with melted butter.

Unwrap the remaining smaller Slab Pastry Crust disk, and roll dough out on a lightly floured work surface into a 12- x 16-inch rectangle. Cut lengthwise into 7 (1 1/2-inch-wide) strips, and arrange in a lattice pattern over peach mixture in crust. Press dough strips to edges to adhere. Whisk together egg and 1 tablespoon water in a small bowl, and brush mixture over dough strips. If desired, sprinkle dough strips with turbinado sugar.

Bake at 375°F until crust is deep golden and peach mixture is bubbly, 45 to 50 minutes, covering pie with aluminum foil during final 15 minutes of bake time to prevent excessive browning. Transfer to a wire rack, and cool completely, about 1 hour.