Prep the Dish and Oven: Preheat the oven to 325°F. Rub a small baking dish (about 2-quart or 8-inch square) with the cut sides of the garlic clove, then grease generously with the butter.

Slice the Potatoes: Peel and thinly slice the potatoes to 1/8 inch thick using a knife or mandoline. Do not rinse or soak the slices—potato starch helps thicken the cream naturally.

Heat the Cream: In a small saucepan, combine the heavy cream, nutmeg, salt, and pepper. Add the halved garlic clove. Warm gently over medium-low heat until steaming (do not boil). Reduce the heat to low and simmer for 2 to 3 minutes; remove from the heat and discard the garlic.

Layer the Potatoes: Arrange half the sliced potatoes in overlapping layers in the prepared dish. Pour in half of the cream mixture. Repeat with the remaining potatoes and cream. Press down gently to level the top and ensure the cream is evenly distributed. The cream should come just below the top layer of potatoes.

Bake: Place the dish on a baking sheet to catch any bubbling overflow. Bake uncovered for 70 to 90 minutes, or until the potatoes are very tender and the top is golden brown. The cream should be thickened but not greasy or curdled.

Rest and Serve: Let the gratin rest for 10 to 15 minutes before serving. This allows the cream to settle and thicken slightly more.