Classic Rich and Silky Potato Gratin

Ingredients

• 1 cup heavy cream
• 2 cups whole milk
• 2 medium cloves garlic, smashed
• 2 small shallots, quartered
• 1/8 teaspoon freshly grated nutmeg
• 6 sprigs fresh thyme
• 2 teaspoons whole black peppercorns
• 1 1/2 teaspoons Diamond Crystal kosher salt
• 3 large russet potatoes
• 2 ounces grated Parmigiano-Reggiano cheese
• 4 ounces grated Gruyère cheese
• 1 tablespoon unsalted butter, softened

Directions

1. Combine the cream, milk, garlic, shallots, nutmeg, thyme, peppercorns, and salt in a small saucepan. Heat the mixture until it boils, then take it off the burner, cover the pan, and let everything infuse for 1 hour.
2. Bring the milk mixture back up to a boil. Strain it into a large heatproof bowl, using gentle pressure on the solids to extract the flavorful liquid.
3. With a mandoline, cut the potatoes crosswise into 1/8 inch slices, letting them fall right into the hot milk mixture. Stir until the slices are completely coated.
4. Heat the oven to 300 degrees F (150 degrees C). In a small bowl, stir together the Gruyère and Parmigiano-Reggiano cheeses.
5. Butter a 2-quart baking dish with the softened butter. Arrange a single, even layer of potato slices in the dish, then scatter some of the cheese mixture over the top. Continue alternating potatoes and cheese until the potatoes are all in the dish and half the cheese mixture is still set aside; the final potato layer should be bare. Pour over any milk mixture left in the bowl.
6. Bake until the potatoes are tender and the top has taken on a pale golden color, about 1 1/2 hours. Sprinkle the reserved cheese mixture over the gratin, change the oven setting to broil, and broil until the surface is browned all over. Let the gratin sit for 30 minutes before serving.