Make Chicken: Season chicken with salt and pepper. Add chicken to a large saucepan and cover with water. Bring to a simmer and cook until just barely cooked through. Remove chicken to a plate and chop into bite-size pieces. Reserve 1 3/4 cups of water.

Chicken Pot Pie Filling: Cook onions, celery, and butter until soft. Stir in flour, salt, pepper, garlic powder, bouillon paste, celery seed, reserved water, and milk. Simmer until thick. Add chopped chicken and frozen vegetables. Season to taste.

Preheat oven to 425°F.

Roll out bottom pie crust and place in pie pan.

Assemble pie: Add cooled chicken pot pie filling. Place second pie crust on top, seal edges, and make a slit in the crust for steam to escape.

Egg wash: Whisk egg with milk and brush a thin layer over the top pie crust.

Bake for 40-50 minutes until golden brown and bubbly. Cover with foil if browning too quickly.

Allow to cool for 15-20 minutes before serving.