If you don’t have a tart pan, unfold pastry sheet onto a lightly greased baking sheet. Bake, on bottom rack, at 400° for 16 to 18 minutes or until puffed and golden. Using a serrated knife, cut a slit around top of pastry, leaving a ½-inch border on all sides. (Do not cut through or to bottom.) Press cut square center to flatten. Spoon onion mixture onto pastry; sprinkle with Parmesan cheese, and bake 5 more minutes.