Preheat oven to 400 degrees. Set a large oven-safe skillet over medium heat. Sauté the onions or garlic in butter, until soft.
Add the celery, carrots, fresh herbs, and seasoning. Cook until soft.
Add in the gravy, and mix in all other vegetables, making sure everything is thoroughly coated in gravy.
Stir in the turkey and let the mixture cool. When the skillet has cooled off, top it with the crust. Crimp and seal the edges. Then cut decorative slits in crust to let the steam escape. Lightly brush the crust with egg wash.
Bake for approximately 40 minutes, longer if needed. Skillet Turkey Pot Pie is done when the crust is your desired color and the gravy is bubbling.
To Make Gravy: Melt 3 tablespoons butter in a skillet. Stir in 1/4 cup flour until it becomes a paste. Slowly whisk in 2 cups of turkey broth and simmer until it starts to thicken. If it gets too thick add a little more broth. If using pre-made gravy, skip this step and simply warm the gravy in the skillet.