Preheat oven to 425. Thaw veggies, bring crusts to room temperature.
Melt butter in large saucepan on medium heat. Add onion and saute until translucent, about 2 minutes. Add flour and stir to incorporate, this is your roux. Gradually stir in broth and milk. Bring to a simmer. Cook until thick and bubbly.
Add veggies, herbs and chicken. Stir to incorporate. Add salt and pepper to taste.
Place one pie crust in bottom of pie plate.
Pour in contents from saucepan.
Top with second pie crust, sealing around edges and flute. Cut several slits in top of crust to vent.
Bake 20 minutes, then cover crust edge with foil to prevent burning. Continue to cook for 20 more minutes until crust is golden brown.