Heat the oven to 425°F and fit one refrigerated pie crust into a standard 9-inch pie plate.

Melt the butter in a saucepan over medium heat. Add the onion and cook for 2 to 3 minutes, until softened.

Stir in the flour, salt, and black pepper, cooking for about 1 minute. Gradually whisk in the chicken broth and milk, then cook and stir for 3 to 5 minutes until thickened and beginning to boil.

Remove the saucepan from the heat and stir in the cooked chicken and slightly thawed mixed vegetables. Allow the filling to cool slightly before placing it in the crust.

Pour the filling into the bottom crust. Cover with the second crust, press or crimp the edges to seal, and cut several slits in the top for steam to escape.

Bake at 425°F for 30 to 40 minutes, until the crust is golden brown and the filling is bubbling. Loosely cover the edges with aluminum foil if they brown too quickly.

Let the finished pot pie stand for 5 to 10 minutes before slicing so the sauce can set and the servings are easier to lift.