How to Make It
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; 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. (Reserve smaller Slab Pastry Crust disk for another use.)
Gently toss together peaches, plums, and ½ cup each of the flour and sugar in a large bowl. Spoon mixture into prepared crust. Stir together butter, cinnamon, salt, and remaining ½ cup each flour and sugar in a medium bowl. Sprinkle over peach mixture in crust.
Bake at 375°F until lightly browned and bubbly, 35 to 40 minutes. Let stand at least 10 minutes or up to 2 hours. Serve warm with vanilla ice cream, if desired.