Preheat the oven to 375°F. On a lightly floured work surface, roll out the pastry sheet (still folded) to an 8-by- 14-inch rectangle. Trim the edges with a sharp paring knife. Transfer to a baking sheet; place in the freezer. Peel, core, and slice the apples 1/4 inch thick. Toss in a large bowl with the sugar.