I've been playing around a little with pastry lately. So when I had some leftover stew, rather than just warm it up, I used it as a pie filling. Came out pretty well.
Roll out half the pastry to line the dish. Bake pie bottom for a few minutes to set slightly, then fill. Make a little thick sauce to pour over. Not too much; just enough to keep it moist. Roll out the rest of the pastry to cover.
Bake in a normal oven until done. Serve.
UPDATE: gently warmed up with some sauted mixed veg, makes a nice looking leftovers of leftovers.