Gratin dauphinois is a gloriousy simple dish of sliced potatoes baked in milk, with a bit of cream and a murmur of nutmeg, until they develop an irresistible browned crusty top simply from the starch content, and without the crutch of cheese. I make it frequently during the colder months, to present as a side or serve on its own with a salad. But over the years, my rendition has evolved into a two-flavor gratin that includes celery root in addition to the potatoes; the knobby root vegetable cooks to a consistency very close to that of spuds, bring a welcome flavor complexity, and makes the dish feel lighter. The trick to a successful potato gratin is first to parboil the potatoes on the stove, until the milk thickens just enough to coat them well, and then finish baking the dish in the oven. This guarantees the potatoes are cooked through, top to bottom.