Garlic and Rosemary Potato Tian

A classic vegetable tian is round, as it takes its name from the Provençal earthenware vessel in which is is made, But for ease, we use a 9-by-13-inch baking dish to make a simple, generously sized potato tian for the holidays. A garlicky broth spiked with whole-grain mustard flavors the butter-tossed sliced potatoes as they cook under foil. The tian is finished with a showering of grated Parmesan and returned to the oven, uncovered, to brown. If you own a mandoline, it makes quick work of slicing the potatoes. The tian can be made in two stages, if convenient. Bake it covered with foil for 45 minutes until tender. At that point, it can be removed from the oven and cooled at room temperature for a couple hours; then, if desired, refrigerate it overnight. About an hour before serving, put the foil-covered tian into a 425°F oven for 20 minutes to rewarm it, then uncover, sprinkle with the Parmesan and bake and cool as directed.