Potatoes and cream transform into a bubbly, rich casserole in this classic French side dish. Use the best cream and Gruyère you can find for the best flavor. Using a mandoline makes quick work of slicing the potatoes, a pro move because the potatoes will oxidize quickly once they’re cut. The volume of your baking dish is key to contain all the potatoes and cream, so don’t use a small dish or it will bubble out into your oven (and do use a baking sheet under the dish).
Heat the oven to 400 degrees. Place a large oval baking dish — about 12 inches long and 2 inches deep — on a foil-lined baking sheet.
In a medium bowl, combine the cream, fine salt and pepper and whisk to combine. Let stand for 5 minutes, then whisk again; this helps to ensure the salt dissolves completely.
Peel the potatoes and place them in a bowl of water to cover. Working with one potato at a time, use a mandoline or sharp chef’s knife to quickly slice the potatoes into thin slices, a little under 1/8-inch (4 millimeters) thick. As you slice each potato, transfer its slices to the baking dish until all the potatoes are sliced and the dish is full. Spread the potato slices out so they nestle in an even layer.
Slowly pour the seasoned cream over the potatoes in the dish; it will look like it won’t all fit, but pour slowly to allow the cream time to seep into every nook and cranny. Using a serrated knife, halve each head of garlic crosswise, then place each half cut side down on the potatoes, spaced equally apart. Cut the butter into small dice and scatter the butter cubes evenly over the surface of the potatoes.
Cover the dish tightly with foil and place in the oven. Bake until the cream is bubbling and thickened and is mostly absorbed by the potatoes and a paring knife inserted in the potatoes slides in and out very easily, about 2 hours. Remove the pan from the oven and discard the foil and garlic halves or save for another use, like mashing on toast.
Heat the broiler to high and position a rack 6 inches away from the heating element. Spread the grated Gruyère evenly over the top of the potatoes and broil until the cheese is bubbly but not crunchy or browned — the cheese should be loose and melty — 2 to 4 minutes, depending on the strength of your broiler. Remove the dish from the broiler and serve immediately.
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