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.
BACON: Thinly slice bacon and add to a large pot over medium-high heat. Cook, stirring frequently, until golden and use a slotted spoon to remove to a paper-towel-lined plate and set aside. Leave grease in the pot (drain off some if there is more than 2 tablespoons). To the bacon grease, add the diced onion, carrots, and potatoes. Saute, stirring occasionally, until veggies are tender, about 10 minutes. They won't get too much more tender throughout cooking so take your time here. Add garlic, saute for another minute or until fragrant.
ROUX: Press veggies to the edges of the pot and add the butter into the center of the pot. Once melted, add in the flour. Cook, stirring constantly for 30 seconds to a minute. Gradually pour in the milk while stirring constantly. Cook until slightly thickened and add in the seasonings -- chicken bouillon powder, dried thyme, and salt & pepper to taste (I add 1/2 teaspoon of each, but you may want less depending on how salty bacon and Parmesan cheese are-- also note I use fine sea salt which is different from table salt). Mix in seasonings. The sauce should be thick enough to coat the back of a wooden spoon. (Note 4)
ADD REMAINING INGREDIENTS: Stir in the corn and chopped rotisserie chicken. Mix to heat through, about 1-2 minutes. Remove pot from heat. Stir in Parmesan cheese and 1/2 cup of the sharp Cheddar cheese. Gently stir until melted. Finally, stir in the reserved bacon. Transfer contents of the pot into prepared pie pan. Sprinkle the remaining 1/2 cup Cheddar cheese on top.
BAKE POT PIE: In a small bowl, whisk the egg with a fork until smooth. Using a pastry brush, brush egg wash around the edges of the pie tin. Place thawed puff pastry on top of the pie and gently press down around corners. Generously brush entire puff pastry with egg wash. Use a knife to cut several vents in the top. Place pot pie on a baking sheet and bake for 35-45 minutes or until deeply golden on top and pot pie is bubbling at the edges. Remove and let stand for 5 minutes before serving. Serve with fresh thyme if desired. Enjoy!