Preheat your oven to 425 degrees F.
Combine your cooked chicken, carrots, peas, green beans, celery, and corn in a large bowl.
Start making your filling for your pot pie. In a large saucepan, melt the butter over medium heat.
Once the butter is melted, add the onion and cook until softened, about 5 minutes.
Stir in the flour and whisk until the butter and flour make a thick paste.
Slowly stir in the broth and milk, stirring constantly as you slowly pour it into the flour and butter mixture.
Add the salt, black pepper, and garlic powder to the filling and allow it to bubble and thicken for 2-3 minutes. Stir occasionally.
Pour the filling over the pot pie ingredients and stir to coat the ingredients.
Pour the pot pie into a lightly greased 9x13 casserole dish.
Lay the cut out pie circles on top of the filling and brush the top of the pie circles with the egg wash. Make the egg wash by combining the white of one large egg and 1 tablespoon of water together and whisking.
Bake for 20-25 minutes, or until the casserole is bubbly and the pie crust is golden brown.
Let cool for 10 minutes and enjoy.