Preheat the oven to 400 degrees F. Melt the butter in a large cast-iron skillet and stir in the onion. Saute over medium heat for 7 minutes; then crumble up the stuffing and add it. Spoon the mashed potatoes here and there in the skillet and scatter in the turkey. Warm the gravy and thin it with a little water. Pour into the skillet, tossing lightly with a fork. Mash the hash down gently with a fork and bake 20 minutes.