Preheat the oven to 425°F
Melt butter in a large skillet over medium heat
Add chopped onion and cook until softened
Stir in flour, salt, pepper, and garlic powder
Slowly whisk in chicken broth and milk
Cook until the mixture thickens
Add cooked, diced chicken
Add mixed vegetables
Stir until well combined
Remove from heat
Place one pie crust in a pie dish
Pour the chicken filling into the crust
Cover with the second pie crust
Seal the edges and cut slits in the top
Brush the top with beaten egg
Bake for 30 to 35 minutes
Let cool for 10 minutes before serving