1Preheat the oven to 350°F. Cut sheet of parchment paper to fit cookie sheet. Unfold puff pastry onto parchment paper. Crimp up the edges with fork to make a square.
2In a small bowl, whisk together egg and cheese. Spread cheese mixture on bottom of pastry. Arrange leek slices on top. Sprinkle with salt and freshly ground black pepper.
3Bake 25 to 30 minutes or until leek slices are golden brown. Remove from oven; allow to cool 1 minute before serving.