Follow package directions to thaw 1 sheet of puff pastry dough. Preheat oven to 350°F. Generously grease a glass 9- or 10-inch pie pan with cooking spray and set aside.
Scrub, dry, and thinly slice (1/8th-inch thick) all the potatoes. In a medium 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 milk and whisk until smooth. Allow to slightly thicken, about 2–3 minutes (it will thicken more with potatoes) and stir in all the seasonings. (Add salt and pepper to taste; I typically add 1/2 teaspoon of each.) Stir in the sliced potatoes, cover the pot with a lid, and reduce 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.
Remove pot from heat and add in the Cheddar cheese. Stir gently until melted. Transfer contents of pot into prepared pie pan. Sprinkle Parmesan cheese evenly over top.
In a small bowl, whisk the 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 sheet pan (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 5 minutes before serving.
Cut and serve. Garnish with fresh thyme if desired. Enjoy!
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Notes
Note 1: I’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). If peeling, thinly slice with a knife or mandolin slicer (1/8th-inch thick) for even quicker prep.Note 2: 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: Packaged grated Parmesan is very salty and won’t melt nicely in this recipe. I highly recommend using a block of Parmesan and a block of sharp Cheddar cheese, grating them on the large holes of the grater. Then measure. Pre-grated cheese contains preservatives that keep the shreds from clumping together in the bag, but also keep the cheese from melting smoothly and will cause a greasy pot pie.Note 4: The puff pastry I recommend can be found in the freezer section of the grocery store, near the frozen bread or desserts. Use a store app if your grocery store has one. I love and recommend Pepperidge Farm puff pastry sheets.Note5: Before adding the filling to the pie pan, ensure it's thickened to your liking. The sauce won't thicken further once the puff pastry is added, so cook the potatoes until creamy. If it's too thick, add a splash of milk.Storage: Once baked, this pot pie is best enjoyed promptly. The puff pastry softens and loses texture over time. Cover leftovers and keep in the fridge for 3–4 days. Reheat in the oven at 350°F, covered with foil, for about 20–25 minutes.