Make the filling
Preheat oven to 400°F with a rack in the center.
Melt butter in a
3.5-quart braiser over medium-high heat. (See note 9 below.)
Add shallot, carrots, celery, and mushrooms. Cook, stirring from time to time, until softened, about 5 minutes.
Add flour and cook, stirring constantly, for one minute. Ensure that flour coats all vegetables somewhat evenly.
Pour in stock, the half cup of cream, salt, pepper, thyme, and sage and stir very well. Add potatoes. Allow liquid to come to a boil, stirring frequently. Then reduce heat and simmer until thickened, about 5 minutes, stirrinng from time to time.
Stir in peas, onions, and cooked turkey. Remove from heat. If you've cooked the filling in an oven-safe braiser, it's ready to top with the crust. Otherwise, transfer the filling to your baking dish now. If you like, you can let the filling cool for a while to make the pastry easier to work with — but you don't have to.
Prepare the crustYou can assemble the crust on a parchment-lined baking sheet first and keep it in the freezer while you make the filling, or you can assemble it directly on the baking dish if you're good at working quickly.
Lightly flour a work surface and carefully unfold the puff pastry. Lightly flour the top too. Roll out the pastry to the dimensions of your baking pan.
Use a very sharp knife or pizza cutter to cut the pastry into approximate 1-inch-wide strips.
Begin by placing the vertical strips. Space them out evenly across the pan or parchment-lined baking sheet to the width of your baking pan.
Here’s how to weave in the horizontal strips. Before placing the first one, lift up and fold back every other vertical strip, then lay down the first horizontal strip. It will be on top of the vertical strips that you left in place. After placing the horizontal strip, replace the vertical strips that you folded back. Those will be on top of the first horizontal strip.
Before placing the second horizontal strip, lift up and fold back the vertical strips that you did not move the first time. Place the second strip and replace the vertical ones.
Continue this process, alternating which vertical strips you move, until the surface of the pan is covered with lattice. If you've prepared the lattice in advance, carefully transfer it from the baking sheet to your filled baking dish.
In a small bowl, beat together the egg and the remaining tablespoon of cream. Quickly but carefully brush the egg wash onto the puff pastry. This will help it become brown and shiny when it bakes.
Bake the pot pie
Place the baking dish onto a rimmed baking sheet to catch any drips.
Bake for 25 minutes, until pastry is puffed and deep golden and filling is bubbly. Let sit for 10 minutes or so before serving.