Preheat the oven to 400°F / 200°C. Place a rimmed baking sheet in the oven while it heats.
Unroll one pie crust and place it in a 9-inch pie dish. Gently press it against the bottom and sides of the dish.
In a large mixing bowl, stir together the Campbell’s Condensed Cream of Chicken Soup, milk, garlic powder, onion powder, dried thyme, black pepper, and salt, if using.
Add the cooked chicken and frozen mixed vegetables. Stir until everything is evenly coated in the creamy sauce.
Spoon the chicken filling into the prepared pie crust and spread it into an even layer.
Place the second pie crust over the filling. Trim excess dough, fold the top edge under the bottom edge, and crimp the edges to seal.
Cut 4 to 5 small slits in the center of the top crust to allow steam to escape.
In a small bowl, beat the egg with 1 tablespoon water. Brush the egg wash over the top crust.
Place the pie dish on the preheated baking sheet. Bake for 30 to 35 minutes, until the crust is deeply golden and the filling is bubbling.
If the edges brown too quickly, cover them loosely with foil during the final 10 minutes of baking.
Remove the chicken pot pie from the oven and let it rest for 10 to 15 minutes before slicing and serving.