PREP: Follow package directions to thaw 1 sheet of puff pastry dough. Preheat the oven to 350 degrees F. Generously grease a glass 9- or 10-inch pie pan with nonstick cooking spray and set aside.
POTATOES: Scrub, dry, and thinly slice (1/8th-inch thick) all the potatoes. In a medium-sized pot over medium-high heat, melt the butter. Briskly whisk in the flour and, whisking constantly, stir for 30 seconds. While whisking constantly, slowly add in the milk and whisk until smooth. Allow to slightly thicken, about 2-3 minutes (it will thicken more with potatoes) and then stir in all of the seasonings. (Add salt & pepper to taste; I typically add 1/2 teaspoon of each.) Stir in the sliced potatoes, cover the pot with a lid, and reduce the heat to medium-low. Allow to cook for 10-15 minutes, stirring occasionally, or until potatoes are fork tender (fork pierces through nicely). Stir in the ham and peas to warm through.
ADD CHEESE: Remove pot from heat and add in the sharp Cheddar cheese. Stir gently until melted. Transfer contents of pot into prepared pie pan. Sprinkle Parmesan cheese evenly over top.
BAKE POT PIE: In a small bowl, whisk an egg with a fork until smooth. Using a pastry brush, brush egg wash around the edges of pie pan. Place thawed puff pastry on top of the pie and gently press down around corners. Generously brush the entire puff pastry with egg wash. Use a knife to cut several steam vents in the top. Place pot pie on a baking sheet (in case anything bubbles over) and bake for 35-45 minutes or until puff pastry is deeply golden on top and pot pie is bubbling at the edges. Remove and let stand for 5 minutes before serving.
ENJOY: Cut and serve. Garnish with fresh thyme if desired. Enjoy!
Note 1: Potatoes: We've made this with peeled and unpeeled potatoes -- didn't make much of a difference IF using Yukon gold potatoes (the peel is so tender/unnoticeable). You can peel if you'd like or save some time and skip the peeling. Thinly slice with a knife or mandolin slicer (1/8th-inch thick) for even quicker prep.Note 2: Ham: Any leftover holiday ham works great and if you don't have any, use pre-cubed ham from the store or a ham steak(s) to quickly chop and add in.Note 3: Cheese: Packaged grated Parmesan is very salty and won't melt as nicely in this recipe. We highly recommend using a block of Parmesan cheese and grating it on the large holes of the grater. Grate a block of sharpCheddar on the large holes as well; then measure. Pre-grated cheese contains preservatives that keep the shreds from clumping together in the bag, but that also keep the cheese from melting smoothly and will cause a greasy pot pie.Note 4: Puff pastry: The puff pastry I recommend in this recipe can be found in the freezer section of your grocery store, near the frozen bread or desserts. It’s a little tricky to find sometimes, so use a store app if your grocery store has one. We love and recommend Pepperidge Farm puff pastry sheets. There are 2 sheets per package and we only use one for this recipe. Set out one to thaw and then tightly wrap up the other and promptly return it to the freezer.Note5: Sauce: Before adding the filling to the pie pan, make sure it is thickened to your desired consistency. The sauce doesn't thicken much (if at all) once the puff pastry is added on, so if needed, keep cooking the saucy potatoes until you have a nice and creamy consistency you are happy with -- it should not be watery or the pot pie will end up watery! On the other end, if it ends up too thick, you may want to add a splash more of milk.STORAGE: Once baked, this pot pie is best enjoyed promptly. Leftovers store okay, but the puff pastry definitely softens and loses texture. Cover leftovers and keep in the fridge for 3-4 days. Reheat in the oven at 350 degrees F, covered with foil for about 20-25 minutes. To prepare ahead of time, make the filling right up to the point of adding the puff pastry on top; instead, let it cool completely to room temperature. Cover the cooled filling in the pie pan and refrigerate. When ready to assemble, top with thawed puff pastry dough and bake. You may need to add an additional 10-15 minutes added to bake time since the filling is chilled. Cover pot pie with foil if the pastry dough is browning too quickly.