Ingredients:

  • 1 tablespoon unsalted butter, softened (for greasing)
  • 1 clove garlic, cut in half (for rubbing the dish)
  • 3 cups (720 ml) heavy cream (35% fat)
  • 1 cup (240 ml) whole milk
  • 4 cloves fresh garlic, smashed and peeled (for infusion)
  • 12 large fresh sage leaves (for infusion)
  • 2 teaspoons kosher salt, plus more for seasoning
  • 1 teaspoon freshly ground black pepper, plus more for seasoning
  • ¼ teaspoon freshly grated nutmeg
  • 4 lbs (1.8 kg) Yukon Gold Potatoes, peeled and very thinly sliced (1/8 inch)
  • 6 oz (170 g) Gruyère Cheese, freshly grated
  • 3 tablespoons unsalted butter (for brown butter drizzle)
  • 6 small fresh sage leaves (for brown butter garnish)

Instructions:

  1. Preheat oven to 375°F (190°C). Generously rub the interior of the 9x13 inch baking dish with softened butter. Rub the cut sides of the halved garlic clove all over the buttered dish, then discard the garlic. Slice the potatoes thinly and uniformly (1/8 inch); do not rinse.
  2. Combine heavy cream, whole milk, smashed garlic cloves, 12 large sage leaves, salt, pepper, and nutmeg in a saucepan. Bring the mixture just to a simmer over medium-low heat; do not boil aggressively. Remove from heat and let steep for 10 minutes. Strain the cream mixture through a sieve into a clean bowl, discarding the solids.
  3. Arrange a tight, single layer of sliced potatoes in the bottom of the prepared dish, overlapping slightly. Season lightly with salt and pepper, and sprinkle evenly with 1/3 of the grated Gruyère. Repeat this layering process two more times, resulting in three potato layers.
  4. Carefully pour the warm, strained cream base evenly over the layers. Gently shake the dish to ensure the liquid seeps all the way down. Cover the baking dish tightly with aluminum foil.
  5. Bake for 60 minutes at 375°F (190°C). Remove the foil and increase the oven temperature to 400°F (200°C). Continue baking, uncovered, for 15–20 minutes, or until the top is deeply golden brown and the potatoes are fork-tender.
  6. Remove the gratin from the oven and let it rest undisturbed for a crucial 15 minutes before slicing and serving. This allows the sauce to redistribute and set.
  7. While the gratin rests, melt 3 tablespoons of butter in a small saucepan over medium heat. Add the 6 small sage leaves. Cook until the butter foams, then subsides, and brown specks form on the bottom. Drizzle the warm brown butter and crisp sage over the top of the gratin just before serving.