Preheat the oven to 400°F and spray a 9-inch round pie dish with cooking spray then set aside. In a small bowl, whisk together the cornstarch and water to make a slurry.
In a large bowl, whisk the slurry, milk, cream of chicken soup, salt, pepper, thyme, and garlic powder until smooth.
Fold the chicken and vegetables into the soup mixture until well combined.
Scrape the filling into the prepared pie dish and spread into an even layer.
Unroll the dough across the pie dish to cover the filling.
Gently pat down the crust to sit on top of the filling. Then, trim the edges of the crust or fold them slightly and crimp to your liking. Add four vent holes to the center of the pie crust.
Brush the entire crust with a light layer of egg wash. Place into the oven and bake for 25-30 minutes until bubbling at the sides and the top is golden brown.
Allow at least 15 minutes to cool and thicken. Then, serve warm.