Uh, this is the best usage of leftover turkey, mashed potatoes and gravy ever. I used the butter crust recipe from Simply Recipes (this made it so much better than it could have been), and then in a skillet sautéed some onions and celery with the leftover turkey in a bit of butter. (Yeah, I know this was just the healthiest lunch ever.) I added the leftover gravy and simmered it all. (I added salt and pepper, and I think when I do this again I'll add some parsley.) Then, after I put in the bottom pie shell, I put in a layer of leftover mashed potatoes, and then scooped the turkey mixture over top. I think I baked it for like half an hour at 400. Oh, and the egg wash made it worth it.
We're going to have more for supper. I highly recommend.