Unfold the puff pastry on a lightly floured surface. Roll into a 10 inch x 14-inch rectangle, then carefully transfer to the prepared baking sheet. In a small bowl, beat together the egg yolk with 1 tablespoon water to create an egg wash (save the egg white for a different use). Lightly brush the entire surface of the puff pastry with the egg wash. With a sharp knife, score a border around the pastry dough that is 1 inch from the edges on all sides, so that you mark a rectangle. With a fork, pierce holes in the pastry all over the center of the rectangle you just created (do not pierce the edges). Each fork imprint should be about 1/2-inch apart. Bake the puff pastry for 10 minutes. It will be golden and slightly puffed. If any areas are very puffy, gently press them down with a fork.
Preheat the oven to 400 degrees F. Line a baking sheet with parchment paper and set aside.