- In a large skillet, brown the mushrooms and onion in the oil. Season with salt and pepper. Deglaze with the vinegar and add the broth. Reduce until almost dry. Let cool. Cover and refrigerate until completely chilled.
- Line a baking sheet with parchment paper.
- Place the dough on a lightly floured surface. Cover with the mushroom mixture. Fold both sides inward, 2.5-cm (1-inch) at a time, towards the centre. Place the roll on the baking sheet and freeze for about 1 hour or until the dough is semi-frozen.
- With the rack in the middle position, preheat the oven to 200 ° C (400 ° F).
- Cut the roll into 24 ½-cm (¼-inch) thick slices. Place flat on the baking sheet, spacing them evenly. Freeze them raw or bake for about 20 minutes or until lightly golden brown. Serve warm or cold.
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.