•Sage & Gruyère Scalloped Potato Gratin•
1/4 cup unsalted butter (1/2 stick), plus more for greasing the aluminum foil, at room temperature
3 cups heavy cream
1/4 cup chopped fresh sage leaves
4 fat garlic cloves, finely grated or minced
1/4 teaspoon freshly grated nutmeg
1 1/2 teaspoons fine sea salt, plus more as needed
5 large eggs
4 pounds russet potatoes (about 6 large or 8 medium)
Freshly ground black pepper
2 1/4 cups grated Gruyère .
Heat oven to 350 degrees, and brush ¼ cup butter on a rimmed 17-by-13-inch baking sheet. Brush one or two pieces of foil (enough to cover the top of the pan) with more butter. Set the foil aside.
In a medium pot, bring the cream, sage, garlic, nutmeg, and a pinch of salt to a simmer. Simmer until reduced by a quarter, about 15 minutes.
In a large heatproof bowl, lightly beat the eggs. Beating constantly, gradually add a little of the hot cream to the eggs, then slowly pour in the rest of the hot cream, whisking to prevent the eggs from curdling. Set aside.
Using a mandoline or a sharp knife, slice the potatoes into ⅛-inch-thick rounds.
Arrange one layer of potatoes on the buttered baking sheet, slightly overlapping the slices. Sprinkle with ¾ teaspoon of the salt, add pepper to taste, then pour half the egg mixture over the potatoes. Top with ½ cup cheese. Repeat the layers of potato, seasoning and egg mixture. Top with the remaining 1 ¾ cups cheese. Cover the baking sheet with the foil (buttered-side down) and bake for 20 minutes. Remove the foil and bake until the potatoes and cheese are browned and bubbling, 25 to 30 minutes. Let cool slightly, then serve.