Preheat your oven to 425°F (220°C).
In a large skillet, melt the butter over medium heat.
Add the chopped onions and cook until translucent, about 5 minutes.
Stir in the flour and cook for 1 minute.
Gradually whisk in the chicken broth and milk, stirring until smooth.
Add the diced chicken, frozen vegetables, thyme, salt, and pepper.
Transfer the filling into a pie dish.
Cover with the pie crust, sealing the edges.
Cut slits in the crust to allow steam to escape.
Bake for 30-35 minutes, until the crust is golden brown.
Let it cool slightly before serving.