How to Make It
Melt 2 tablespoons of the butter in a large skillet. Add the onion, celery and carrot and cook over low heat until softened, about 12 minutes. Add the turkey and cook, stirring, until heated through. Add the gravy and cook until bubbling. Remove from the heat, stir in the parsley and sage and season with salt and pepper. Transfer the filling to a 9-by-12-inch baking dish and let cool.
Preheat the oven to 400°. In a bowl, blend the mashed white and sweet potatoes with the chives. Season the mashed potatoes with salt and pepper and beat in the egg. Spread the potatoes over the filling and brush the top with the melted butter. Bake in the upper third of the oven for about 30 minutes, or until the topping is lightly browned and the filling is bubbling. Let the pie stand for about 10 minutes before serving.