Preheat oven to 350° and pull the pie crust from the refrigerator to soften.
Heat the butter in a large pan over medium heat, then add the onion and saute until translucent.
Add the flour, garlic powder, salt, and pepper and stir continuously for about 30 seconds, toasting the flour.
Add the broth and half n half, and whisk until no clumps of flour remain. Cook over medium heat until thickened, about 5 minutes.
Add the thawed vegetables and cooked chicken and heat through. Remove from heat.
Roll out the pie crusts and grease a 9-inch pie pan. Transfer one pie crust to the pie pan and pour the chicken filling into it.
Carefully place the second pie crust over the filling. Seal the edges and trim any excess dough. Cut 4 slits in the top of the pie crust to allow steam to escape.
Whisk together the egg and 1 tbsp cold water, and brush the mixture over the top of the pie.
Bake in the preheated oven for 40-45 minutes, or until bubbly in the center and the crust is golden brown. Serve!