Preheat oven to 400°F.
In a saucepan, cover the potato and carrots with water, bring to a boil, then turn down and simmer for about 4 to 6 minutes until fork tender. Take off the heat and drain. Set aside.
In the meantime, chop the chicken, onions and celery.
In a large, deep frying pan, sauté the chicken, onions, and celery in butter until the chicken is cooked and the onions are translucent.
Add the diced carrots and potatoes, then stir in the flour, poultry seasoning, garlic powder, celery seed, salt and pepper.
Add the chicken broth and milk. Simmer, stirring occasionally, until thickened, about 4 to 7 minutes. If you are having trouble thickening the sauce, add in 1 to 2 Tablespoons of cornstarch. The mixture will thicken as it bakes in the oven, but it should be slightly thickened at this point.
Place the bottom pie crust in a 9 inch pie plate.
Making sure the filling is hot, pour it into the pie crust inside of the pie plate.
Quickly cover with top crust, sealing the edges and cutting away extra dough.
Beat the egg and water to make an egg wash and brush it onto the top of the pie crust.
Make four small slits in the top crust to help steam escape.
Bake for 30 to 35 minutes, covering edges with foil if necessary. The pot pie is done when the pie crust is golden brown and the filling is bubbly.
Cool for 10 minutes before serving. Best when served warm.