Lightly season the chicken breasts with salt and pepper, then place in a pot and cover with water. Simmer for about 12 to 15 minutes, or until cooked through. Remove and chop it into bite-sized pieces, reserving about 1 ¾ cups of the cooking water for later.

In the same pot, melt the butter and add the onions and celery. Sauté until soft. Stir in the flour, salt, pepper, garlic powder, celery seed, and chicken bouillon. Gradually pour in the reserved broth and milk, stirring until thick and creamy. Mix in the cooked chicken and frozen vegetables, then let cool slightly.

Roll out the bottom crust into a 9-inch pie pan. Transfer the cooled filling into the crust, spreading it evenly. Roll out the top crust and place it over the filling. Seal the edges and cut small slits for steam to escape.

Brush the top crust with the egg wash. Bake in a preheated oven at 425°F for 40 to 50 minutes or until golden brown. If browning too quickly, cover with foil.

Allow the pie to cool for about 15 to 20 minutes before serving, then enjoy warm.