Cook 2 minutes, stirring often, until onion is browned and caramelized. Let stand until cool, about 5 minutes. Roll puff pastry on a lightly floured work surface to a 14- x 12-inch rectangle.

Transfer the onion to a bowl; season with salt and pepper. Line a baking sheet with parchment paper. On a lightly floured work surface, roll out the puff pastry to an 11-inch square. Using a plate ...