Preheat your oven to 425°F (220°C).
In a saucepan, melt butter over medium heat.
Add chopped onion, cooking until translucent.
Stir in flour and cook for about 1 minute.
Gradually add chicken broth and milk, stirring constantly.
Cook until the mixture thickens, about 5 minutes.
Add shredded chicken, carrots, peas, potatoes, salt, pepper, and thyme.
Remove from heat and let the filling cool slightly.
Roll out one pie crust and place it in a pie dish.
Pour the chicken filling into the crust and cover with the second crust.
Seal edges and cut slits in the top for steam to escape.
Bake for 30-35 minutes or until golden brown.