How to Make It
Preheat oven to 375°F. Unwrap the larger Slab Pastry Crust disk, and roll dough out on a lightly floured work surface into an 18- x 13-inch rectangle. Wrap dough around rolling pin; gently fit inside a 15- x 10-inch jelly-roll pan. Turn edges under; crimp. Prick bottom and sides of dough with a fork. Line with parchment paper, leaving a 3- to 4-inch overhang around pan edges. Fill with pie weights or dried beans. Bake 10 minutes. Remove parchment and pie weights. Return crust to oven; bake at 375°F for 5 minutes. Transfer to a wire rack; cool completely, about 30 minutes.
Gently stir together berries, lemon zest, and lemon juice in a large bowl. Stir together granulated sugar, brown sugar, flour, thyme, and salt in a small bowl. Sprinkle over berry mixture; stir gently to incorporate. Spoon berry mixture into prepared crust. Sprinkle evenly with cubed butter.
Unwrap remaining smaller Slab Pastry Crust disk; roll dough out on a lightly floured work surface into a 12- x 6- x ¼-inch rectangle. Cut 15 (12-inch-long, 1/4-inch-wide) strips from dough rectangle. (Discard dough scraps, or reserve for another use.) Whisk together egg and 1 tablespoon water in a small bowl.
Braid 3 dough strips together, keeping the braid as snug as possible. Carefully place dough braid crosswise on top of pie about 2 inches from 1 short side of pan. Brush top of crimped crust where dough braid overlaps with it, and press together to adhere. Brush top of dough braid with some of the egg mixture. Repeat process with remaining 12 dough strips and egg mixture, making 5 dough braids total, each spaced 2 to 3 inches apart on top of pie.
Bake at 375°F until crust is golden brown and filling is bubbly, 45 to 50 minutes, covering crust edges with aluminum foil during final 15 minutes of baking to prevent excessive browning, if needed. Transfer to a wire rack, and cool completely, about 1 hour.