This gratin recipe is adapted from this one in Fine Cooking. I use a lot less thyme and about half as much parmesan. I've also been known to throw a little cream in there if I'm in the mood. The caramelized onions on the bottom are really what make this dish incredible. They add a whole 'nother dimension if you know what I'm saying. This dish is super simple to construct,and the levels of flavor that are produced are well worth the 70 minute baking time.