Make the Piecrust Dough: In a large mixing bowl, whisk together the flour and salt. Cut the butter into 7 or 8 pieces and drop them into the flour. Add the vegetable shortening too.
With clean hands, lightly toss the mixture, breaking up the pieces of butter and vegetable shortening to about half their original size.
Now use a pastry blender to turn the mixture into coarse crumbs about the size of baby peas. When you're finished, the dough should be evenly moist.
Using a measuring spoon, sprinkle the cold water over the dough. With a big fork, stir the mixture, pressing down on it occasionally, until it packs together. If it continues to seem a little dry, add 1 or 2 more teaspoons of water.
Gather up the dough and pack it like a snowball. On a lightly floured surface, gently knead the dough once or twice. Then put the dough on a long sheet of plastic wrap and press it into a disk about 1-inch thick. If the edges of the disk crack, just pinch them back together. Wrap the disk and refrigerate it for 1 hour while you make the filling.
Prepare the Filling: Melt the butter in a large saute pan over medium-low heat. Stir in the onion and celery. Cover the pan and gently sweat the vegetables for about 8 minutes, stirring occasionally.
Then stir in the flour. Increase the heat slightly and continue to cook and stir the mixture for 1 more minute.
Add the stock to the pan, whisking to evenly blend all of the ingredients. As the stock starts to thicken, whisk in the milk, sage, thyme, and salt and pepper. Depending on the saltiness of the stock you've used, you will likely need about 1/2 teaspoon of salt. Start with a little less, then taste the sauce and add more if needed.
Stir in the turkey and vegetables and simmer the mixture, stirring often, for 2 minutes. Remove the pan from the heat.
Generously butter a 2-quart round, shallow casserole (about 8 1/2 to 9 1/2 inches with sloping sides). Transfer the filling to the casserole and smooth the top with a spoon. Let the filling cool for about 15 minutes.
Meanwhile, heat the oven to 400 degrees F.
Roll the Crust and Top the Pie: While the oven heats, roll the pastry for the top crust. Put a sheet of waxed paper (about 12-inches long) on your work counter and lightly flour it.
Put the pastry on the waxed paper and dust it and your rolling pin with flour. Roll the dough into an 11-inch disk. Start in the middle and push the pin out toward the edge of the disk, turning the paper as needed to roll the dough evenly.
Invert the crust, with the paper, so it is centered atop the filling. Peel away the waxed paper and gently fit the edge of the crust into the dish.
In a small bowl, whisk the egg yolk and milk for the glaze. Lightly brush the glaze onto the crust. Then use a fork to poke 2 or 3 steam vents in the pastry.
Bake the potpie on the center oven rack for 40 minutes, until well browned. Slide an aluminum foil-lined baking sheet onto the shelf below the pie, to catch any spills.
Transfer the potpie to a cooling rack and cool for 10 minutes before serving.