You can buy puff pastry dough squares, already rolled-out, in the frozen section of some grocery stores. If you buy puff pastry sold in blocks, roll it out into a 3-mm (⅛-inch) thick square. You can freeze the mushroom pastry slices on a baking sheet lined with parchment paper and then transfer them to a sealable plastic bag. Keep them in the freezer until ready to bake. The baking time will increase to approximately 25 minutes.