1-First Steps: Prep and Cook the Chicken: Begin by seasoning the chicken breasts with salt and pepper. Place them in a saucepan, cover with water, and simmer until just cooked through, about 10-15 minutes depending on thickness. For more details on cooking moist chicken, visit this guide to cooking tender chicken. Once done, remove the chicken, let it cool, then chop it into bite-size pieces and reserve about 1 ¾ cups of the cooking water.

2-Next: preheat your oven to 425 degrees F and prepare your other ingredients. This prepping helps reduce stress, as mentioned in the recipe tips for making the filling ahead of time.

3-Making the Filling: In the same saucepan, cook the onions, celery, and butter until soft. Stir in the flour, salt, pepper, garlic powder, bouillon paste or cube, and celery seed to create a roux. Gradually add the reserved cooking water and 1 cup milk, stirring until the mixture thickens into a creamy gravy.

4-Then, add the chopped chicken and 8 ounces frozen mixed vegetables to the filling. Season to taste and let it cool, which makes assembly easier. This step ensures the filling sets up well for baking.

5-Assembling and Baking: Roll out one portion of the pie dough to fit a 9-inch pie pan and place it in the pan. Add the cooled filling into the pie shell, then roll out the second portion of dough and place it over the filling. Seal and crimp the edges, and cut slits in the top crust to let steam escape.

6-Whisk the egg with 1 tablespoon milk and brush it over the top crust for a golden finish. Bake for 40-50 minutes until the top crust is golden brown and the filling is bubbly. If the crust browns too quickly, cover with foil to prevent burning, and let the pie cool for 15-20 minutes before serving.