Before starting, be sure the puff pastry is out of the freezer thawing while you prepare the filling of the pot pie. Preheat the oven to 425F and heat a large ovenproof skillet to medium heat on the stovetop.
Add 2 tablespoons of the butter and the onion, celery, and carrots to the skillet with the salt and several grinds of pepper. Stir for about 5 minutes to soften the veggies and release some of the liquid.
Sprinkle the flour all over the vegetables and stir for one minute more, then slowly stream in the stock, stirring the entire time to combine the flour with the stock and thicken the mixture.
Let the stock mixture come to a bubble, then stir in the cream, turkey, and peas. Taste for seasoning and add more salt and pepper to your preference, then turn the heat off the skillet.
If needed, stretch or roll the puff pastry, using flour to prevent sticking, to roughly the size of the dish you are using. Drape the pastry over the entire dish, pressing the edges to the baking dish to prevent it from falling into the mixture.
Melt the remaining tablespoon of butter and brush it over the puff pastry. Cut a small slit in the pastry to allow steam to escape. Place the entire dish in the oven and bake for 30 minutes or until the pastry is golden brown and puffy all over.
Let the pot pie rest for 10 minutes then use a large spoon to scoop servings of pot pie and puff pastry into shallow bowls. Enjoy!