Parmentier is France's family dish par excellence: a layer of tender meat, a layer of mashed potatoes, a golden top. Made with the Christmas bird it's even better, because the roast meat already has all its flavor and the roasting juices keep it moist.
Two things make the difference: a properly moist filling (poultry reheated dry turns stringy) and potatoes mashed by hand, never in a blender. And since these are leftovers, the fridge rules apply to the letter: the recipe reminds you of them.
Good to know · Cooked holiday poultry must go into the fridge within 2 hours of the meal, well covered, and be eaten within 2 days. Reheat it only once, until piping hot all the way through: this pie belongs to the evening of the 25th or the 26th, not the 28th.