1-First, season the chicken breasts with salt and pepper, place them in a saucepan, and cover with water. Simmer until just cooked, then remove and chop into bite-size pieces; reserve about 1 ¾ cups of the cooking water. This step ensures the chicken is tender and flavorful, setting the base for the filling.

2-Next, in the same pan, cook the sliced celery, chopped onion, and butter until soft. Stir in the flour, 1/2 teaspoon salt, 1/4 teaspoon pepper, 1/4 teaspoon celery seed, 1/2 teaspoon garlic powder, and 1 teaspoon bouillon paste. Slowly add the reserved water and 1 cup milk, simmering until the mixture thickens for a rich sauce.

3-Assembling and Baking: Add the chopped chicken and 8 ounces frozen mixed vegetables to the thickened sauce; stir well and let the filling cool completely. Preheat your oven to 425 degrees F, roll out one portion of the pie dough to fit a 9-inch pie pan, and place it inside. Pour in the cooled filling, then cover with the second dough portion, sealing and crimping the edges while cutting slits for steam.

4-Whisk 1 egg with 1 tablespoon milk for an egg wash and brush it over the top crust. Bake for 40-50 minutes until golden brown and bubbly; cover with foil if the crust browns too fast. Let the pie rest for 15-20 minutes before serving to allow the filling to set.