cup plus 2 tablespoons heavy whipping cream
large garlic clove, peeled
pounds russet potatoes, peeled, cut into ⅛-inch-thick rounds
teaspoons chopped fresh thyme
cup whole milk
cup coarsely grated Gruyère cheese
* Position rack in center of oven and preheat to 350°F. Generously butter 9-inch-diameter glass pie dish. Place pie dish on rimmed baking sheet. Combine cream and garlic in small saucepan and bring to simmer over low heat. Cover and remove from heat. * Arrange 1/3 of potato rounds, slightly overlapping, in concentric circles in prepared pie dish; sprinkle with 1/2 teaspoon thyme and season lightly with salt and pepper. Pour 1/4 cup garlic cream over. Repeat layering 2 more times with remaining potato, thyme, salt, pepper, and garlic cream. Pour remaining garlic cream over, then 1/4 cup milk. Sprinkle cheese over. * Bake gratin until golden on top and potatoes are very tender, 1 hour to 1 hour 10 minutes. Let potato gratin rest 10 minutes before serving.