A wonderful dish of French origin, typically made with multiple layers of finely sliced potatoes (though other vegetables could be used), milk or cream and garlic. The name has nothing to do with gratitude (although it should); to gratinate is to cook a dish in such a way that a crust forms, often by sprinkling with cheese or breadcrumbs and grilling.
Au gratin recipe
Potato, celeriac, carrot and swede gratin