Season the chicken breasts with salt and pepper. Place the chicken in a large saucepan and submerge in water. Bring to a gentle boil and cook until just cooked through. Remove the chicken and let rest briefly before cutting into bite-sized cubes. Reserve approximately 1 ¾ cups of the cooking water, discarding the remainder.

In the same saucepan, melt the butter and sauté the chopped onion and sliced celery until softened and translucent. Sprinkle in the flour, salt, pepper, garlic powder, bouillon paste, and celery seed. Gradually add the reserved cooking water along with the milk, stirring constantly over medium-low heat until thickened.

Incorporate the diced chicken and frozen vegetables, tasting and adjusting seasoning with additional salt, pepper, bouillon, or garlic powder if necessary. Allow the mixture to cool sufficiently so it is no longer hot when added to the crust.

Preheat the oven to 425°F (220°C).

On a lightly floured surface, roll out one portion of the chilled pie dough into a circle approximately 12 inches wide. Gently transfer it into a 9-inch pie pan, pressing it into place.

Pour the cooled filling into the prepared crust. Roll out the second portion of pie dough and position it over the filling. Pinch the edges of the crusts together to seal, crimping if desired. With a sharp knife, cut a small vent in the top crust to allow steam to escape.

Whisk the egg with the tablespoon of milk to create an egg wash, brushing it lightly over the top crust.

Bake for 40–50 minutes, or until the top is golden brown and the filling is bubbling. If the crust begins to brown excessively, shield it loosely with aluminum foil.

Let the pot pie rest for 15–20 minutes before slicing and serving, allowing the filling to set.