Preheat the oven to 450 degrees F. Caramelize the leeks by cooking them in 1 tablespoon olive oil and a pinch of salt and pepper in a large skillet over medium heat for 10 minutes.
Roll out the puff pastry into a 20 by 10-inch rectangle, move the pastry to a baking sheet and start by layering the potato slices on one half of the rectangle (we are going to fold the other empty half over and on top in order to seal it up, sort of like a calzone). Season the potatoes with salt and pepper, then add the crumbled bacon and leeks. Top with the Parmesan and seal up the puff pastry square, then crimp the edges. Brush the top of the puff pastry with the egg and cut 3 slits in the top of the puff pastry to allow the steam to escape.
Bake for 20 minutes until golden brown and puffed. Serve warm.
Baking the potatoes, leeks, Parmigiano, and bacon inside the puff pastry simulates the "en papillote" method of cooking, so everything steams together inside and the flavors blend into one. The wonderful aromas stay trapped inside until you take that first bite of the puff pastry and the smells waft up to your nose. Yum.
Recipe courtesy of Joanne Maurer Ozug for 2011 Cooking Channel, LLC. All Rights Reserved.