Steam-Baked Two-Potato Gratin

Butter an 8-inch square baking dish or a 9-inch round baking dish.

In a small saucepan, combine the heavy cream, garlic, thyme, salt, and pepper. Bring to a simmer over medium heat. Once the cream mixture comes to a simmer, remove from the heat and whisk in the Gruyère cheese. Let steep uncovered for 15 minutes.

Starting with the sweet potatoes, peel and thinly slice ⅛-inch thick using a mandoline slicer. Repeat with the russet potatoes.

When the cream has finished steeping, start by pouring 2 tablespoons of the cream mixture into the bottom of the buttered baking dish. Shingle a layer of russet potatoes to cover the bottom of the baking dish. Pour ¼ cup cream mixture over the potatoes and follow with a layer of sweet potatoes. Pour ¼ cup cream mixture over the sweet potatoes and repeat until you’ve used up all of the potatoes and cream. Using your hands, press the potatoes down into the dish to submerge them under the cream mixture.

Transfer the baking dish to the oven and cook until the potatoes are tender when pierced with a knife and the top is golden brown in spots, 30 to 40 minutes.

Remove from the oven and let rest for 15 minutes before serving.