Preheat Your Oven. Start by preheating your oven to 425°F.
Prepare the Pie Crusts. Place one pie crust in the bottom of a 9-inch pie dish. Set the second crust aside for the top.
Cook the Filling. In a large saucepan, melt the butter over medium heat. Add the chopped onions and cook until they become translucent.
Create the Roux. Stir in the flour, salt, and pepper until well combined, creating a roux. Slowly add the chicken broth and milk, stirring continuously until the mixture thickens and becomes smooth.
Add the Turkey and Vegetables. Add the diced turkey, mixed vegetables, and dried thyme (if using) to the saucepan. Stir until the filling is well mixed and heated through.
Fill the Pie Dish. Pour the turkey and vegetable filling into the pie dish over the bottom crust.
Cover with the Top Crust. Place the second pie crust over the top of the filling. Seal the edges by crimping them together, and make a few small slits in the top crust to allow steam to escape.
Bake to Golden Perfection. Bake in the preheated oven for about 30-35 minutes, or until the crust is golden brown and the filling is bubbling.
Cool and Serve. Allow the turkey pot pie to cool for a few minutes before slicing and serving.