Prepare the Pot Pie Pastry. Cover and chill in the refrigerator while preparing the filling.
In a large saucepan, melt butter over medium heat. Add celery and onion and cook until onion is tender but not brown, stirring occasionally. Stir in flour, poultry seasoning, and salt. Stir in stock all at once. Cook and stir over medium heat until thickened and bubbly. Stir in turkey. (For make ahead, cool filling and store, covered, in the refrigerator up to 3 days.)
Lightly grease a 15x10x1-inch baking pan or line pan with foil; set aside.
On a lightly floured surface, divide 1 pastry portion into two balls, one 1/3 of the portion, the other 2/3 of the portion. Roll the larger ball to a 7-1/2-inch circle and the smaller ball to a 4-3/4-inch circle; set aside. Repeat with remaining 3 pastry portions.
Line a 4-inch disposable foil pie pan or a 10-ounce custard cup with one of the 7-1/2-inch pastry circles. Spoon one-fourth of the filling into pastry-lined pie pan or cup. Top with one of the 4-3/4-inch pastry circles.
Lightly moisten edges with water and bring up the edges of the lower dough, crimping over top layer. Carefully turn the filled pie pan upside down onto prepared baking pan; remove the pie pan. Use a fork or wooden toothpick to prick the top of the pastry several times. Using the same pie pan, repeat with remaining pastry circles and filling. Brush tops and sides of the filled pies with egg white.
Bake in a 425 degree F oven about 25 minutes or until crust is golden brown. Serve with Turkey Gravy and, if you like, bread dressing and mashed potatoes.
In a small saucepan, melt butter over medium heat. Stir in all-purpose flour until smooth. Stir in turkey stock or chicken broth all at once. Cook and stir over medium heat until thickened and bubbly. Cook and stir 1 minute more. Season to taste with salt and ground black pepper.
In a very large mixing bowl, combine flours and salt; add shortening. Beat with an electric mixer on low speed until pieces are the size of small peas. Add water and egg yolk; beat at low speed just until a dough begins to form (about 15 to 20 seconds).
Form dough into a ball with hands. Divide into 4 portions. Using your hands, gently flatten each portion. Tightly wrap portions with plastic wrap and chill for at least 30 minutes.
Continue as directed in Step 4 in Strongbow Turkey Pie recipe.