Preheat oven to 400°F.
Shred or dice the cooked turkey and set aside.
Melt the butter in the cast iron skillet over medium-high heat. Once the butter has melted add onions. Sauté for about 5 to 6 minutes or until onions are tender.
Sprinkle in flour and cook for 1 minute, stirring constantly. Add broth, cream, salt, black pepper, and poultry seasoning. Cook, stirring frequently until it comes to a simmer and begins to thicken. Simmer for additional 2 minutes and remove from heat.
Add the turkey, parsley, and frozen vegetables to the cast iron skillet and stir well until combined. Set aside.
On a lightly floured work surface roll the thawed puff pastry into an approximately 12x12-inch square
Carefully drape the puff pastry over the cast iron skillet then trim the extra pastry leaving a 1-inch border all around. Fold the edge of the puff pastry under itself and crimp the edges with a fork to seal.
Cut 3 slits into the top of the puff pastry. Whisk together the egg white and evenly brush it over the puff pastry crust.
Bake for 25-30 minutes or until the crust is golden brown.
Allow to rest for 10 minutes before serving.