Step 1

Arrange a rack in the middle of the oven and heat the oven to 350°F. Coat a 9x13-inch baking dish or other 4-quart baking dish with 1 tablespoon unsalted butter. Grate 8 ounces Gruyère cheese on the large holes of a box grater (about 2 cups).

Step 2

Peel 2 1/2 pounds Yukon Gold potatoes, remove any blemishes, slice into 1/4-inch thick rounds, and place into a large pot. Mince 3 garlic cloves. Add the minced garlic, 2 cups heavy cream, 2 cups whole milk, 1 teaspoon kosher salt, 1 teaspoon black pepper, 1/2 teaspoon nutmeg, and 2 dried bay leaves. Bring to a simmer over medium heat. Simmer, stirring often, until the potatoes are fork tender but still slightly firm, 5 to 7 minutes. Remove and discard the bay leaves.

Step 3

Using a slotted spoon, transfer half of the potatoes into the prepared baking dish and arrange in an even layer. Top with half of the grated cheese. Arrange the remaining potatoes on top of cheese in an even layer. Pour about 2 cups of the cream mixture over the potatoes, stopping just below the top layer. (Do not use all of the cream mixture.) Top with the remaining cheese.

Step 4

Bake until the potatoes are golden brown and easily pierced with a knife all the way to the bottom of the baking dish, 40 to 45 minutes. Garnish with 1 teaspoon fresh thyme leaves and let cool at least 15 minutes on a wire rack before serving.