Preheat your oven to 425 ℉ (220°C). While the oven is heating, combine the cubed chicken breast, sliced carrots, frozen green peas, and sliced celery in a large saucepan. Cover the mixture with water, bring it to a boil, and let it cook for 15 minutes. Afterwards, drain thoroughly and set the mixture aside.

In a separate saucepan over medium heat, melt the unsalted butter and add the chopped onion. Sauté until the onion becomes tender and transparent. Meanwhile, combine the all-purpose flour, salt, black pepper, and celery seed in a small bowl. Gradually stir the flour mixture into the onions and butter. Slowly add the chicken broth and milk, stirring constantly to avoid lumps. Simmer over medium-low heat until the sauce thickens. Remove from heat when done. I like to let the sauce simmer just a little longer for an even richer flavor.

Lay one of the unbaked pie crusts into a 9-inch pie pan, pressing it firmly into the bottom and up the sides. Add the cooked chicken and vegetable mixture (from Step 1) evenly over the crust. Then pour the hot, thickened sauce (from Step 2) over the chicken and vegetables, making sure it spreads well and fills the gaps. Cover with the second pie crust, seal the edges together, and trim any excess dough. Cut a few small slits in the top crust to allow steam to escape during baking.

Place the assembled pie into the preheated oven and bake for 30 to 35 minutes, or until the pastry turns golden brown and the filling is bubbling. If the edges start to brown too quickly, you can cover them with strips of foil to prevent burning.

Remove the pie from the oven and let it cool for at least 10 minutes before slicing and serving. This cooling period helps the filling to set and makes serving much easier. Personally, I love waiting just those extra few minutes—the flavors meld together beautifully!