Preheat the oven to 375°F, and lightly grease a 2.5-quart baking dish. Set aside. Remove the pastry sheet from the freezer, and allow to thaw slightly for easy handling. 

Heat a large skillet over medium heat. Add the butter, onion, and mixed vegetables. Cook for 4 to 5 minutes until the onions begin to soften, then add the garlic and thyme. Cook for an additional 30 seconds, then add the flour.

Cook the flour for an additional 2 to 3 minutes, then slowly whisk in the chicken broth and milk. Taste, and season with salt and pepper. Continue to stir until slightly thickened, then add the turkey.

Transfer the mixture into the prepared baking dish. Roll out the puff pastry on a lightly floured surface just enough to remove creases. 

Place the puff pastry directly over the baking dish (there will be some hanging over the edges), or trim the edges and place the puff pastry just inside the dish on top of the filling.

Carefully cut 3 to 4 slits into the puff pastry, and apply the egg wash using a pastry brush.

Bake for 30 to 35 minutes or until the pastry is golden brown. Cool for 5 to 10 minutes before serving.

Store leftovers in the refrigerator for up to 3 days.