Heat oven to 375° F. Cook the bacon in a skillet over medium heat until crisp, 6 minutes. Transfer to a paper towel-lined plate. Discard all but 1 tablespoon of the drippings and return to medium heat. Add the leeks and cook for 3 minutes. Unfold the puff pastry and place on 2 parchment-lined baking sheets. Sprinkle with the cheese and scatter the leeks and bacon over the tops. Bake until the pastry is golden brown and crisp, about 25 minutes.
