Sweet Potato and Gruyère Gratin

Step 1

Heat oven to 400 degrees and generously butter a 9-by-13-inch baking pan or shallow gratin dish. Butter a piece of foil large enough to cover top of pan. Sprinkle Parmesan all over bottom of pan.

Step 2

In a medium pot, bring cream, sage, rosemary, garlic, nutmeg and a pinch of salt to a simmer. Simmer until reduced by 1/4, about 10 minutes.

Step 3

In a large, heatproof bowl, whisk eggs just enough to break them up. Slowly pour hot cream into eggs to combine, whisking while pouring, and reserve the mixture.

Step 4

Meanwhile, using a mandoline or sharp knife, slice the potatoes into 1/8-inch-thick rounds.

Step 5

Place 1 layer of potatoes in the pan, slightly overlapping as you go, using about a third of the slices. Sprinkle with 1/4 teaspoon salt and pepper to taste, then pour 1/3 of the egg mixture over potatoes. Top with 1/2 cup Gruyère. Repeat with another layer of potatoes, 1/2 teaspoon of salt, pepper and 1/3 egg mixture. Top with 1/2 cup Gruyère. Top with remaining potatoes, 1/4 teaspoon salt, pepper to taste, and remaining egg mixture (but not the remaining cheese). Press down to compact the potatoes. Cover with foil and bake until potatoes are tender, about 40 minutes, then remove foil, sprinkle top with remaining 2/3 cup Gruyère and bake until browned and bubbling, 25 to 30 minutes. Let cool slightly, then serve.