Bring a pot of water to a boil and add the chicken breasts with salt. Simmer until the chicken is fully cooked through (about 12-15 minutes), then remove and set aside to cool slightly. Once cooled enough to handle, shred the chicken into bite-sized pieces and set aside. Measure out and reserve 1¾ cups of the cooking liquid—this will become the base of your filling and adds wonderful chicken flavor.

In a large saucepan, melt the butter over medium heat, then add the diced onion and celery. Cook for 4-5 minutes until softened and fragrant. Sprinkle the flour over the vegetables and stir constantly for 1-2 minutes to create a roux—this flour-and-fat mixture will thicken your sauce. Add the garlic powder, celery seed, black pepper, salt, and chicken bouillon paste, stirring well to combine. I find the bouillon paste adds such savory depth that you really notice it once everything comes together.

Gradually pour in the reserved 1¾ cups of chicken cooking liquid while stirring constantly to avoid lumps, then add the milk and continue stirring until smooth. Simmer over medium heat for 3-4 minutes until the sauce thickens enough to coat the back of a spoon—you want it creamy but not watery. Stir in the shredded chicken from Step 1 and the frozen mixed vegetables, then taste and adjust seasonings as needed. Remove from heat and let the filling cool to room temperature (about 15-20 minutes)—this is important because a cool filling won't make the crust soggy during assembly.

Preheat your oven to 425°F. While it heats, take one piece of pie dough and roll it out to about 12 inches in diameter on a lightly floured surface. Transfer it to a 9-inch pie pan, letting any excess hang over the edges—this will be trimmed after you add the filling.

Pour the cooled filling from Step 3 into the crust, spreading it evenly. Roll out the second piece of pie dough to about 11 inches and place it over the filling. Trim both crusts to a 1-inch overhang, then fold the edges under and crimp with a fork or your fingers to seal. Cut a few small slits in the top crust to allow steam to escape during baking—this prevents the pie from becoming soggy on top.

In a small bowl, whisk together the egg and 1 tablespoon of milk to create an egg wash. Brush this mixture evenly over the top crust—this gives the pie a beautiful golden-brown finish. Place the pie on a baking sheet to catch any drips and bake for 40-50 minutes until the crust is golden brown and the filling bubbles slightly at the edges.

Remove the pie from the oven and let it rest for 15-20 minutes before slicing. This resting time allows the filling to set slightly so it won't run all over the plate when you cut into it. Serve warm with a simple green salad or roasted vegetables on the side.