Ingredients

• 1/4 cup soy sauce
• 2 tablespoons porcini mushroom powder (see Cook's Note)
• 1 1/4 cups heavy cream
• 3 cloves garlic
• 3 sprigs fresh thyme
• 1/4 cup creme fraiche
• 1/4 teaspoon ground nutmeg
• Kosher salt and freshly ground black pepper
• 2 pounds Yukon gold potatoes, thinly sliced on a mandoline (or with a very sharp knife)
• 2 cups grated Gruyere cheese

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Instructions

1. Watch how to make this recipe.
2. Special equipment: a mandoline, optional
3. Preheat the oven to 375 degrees F. Stir together the soy sauce and mushroom powder in a small bowl and set aside.
4. Add the cream, garlic and thyme sprigs to a medium pot and set over medium-high heat. Bring to a simmer, cover and remove from the heat. Let stand 15 to 30 minutes.
5. Remove the garlic and thyme from the cream, then whisk in the creme fraiche, nutmeg and soy-mushroom mixture until well incorporated; season with salt and pepper.
6. Assemble the gratin by putting half of the potatoes in a single overlapping layer in the bottom of a 2-quart baking dish. Coat with half of the cream mixture and sprinkle on 1 cup of the Gruyere. Repeat with the remaining potato slices, cream mixture and 1 cup Gruyere. Cover with foil and transfer the dish to a baking sheet.
7. Bake until the potatoes are easily pierced with a knife, about 1 hour. Remove the foil and continue to bake until the top is golden brown and bubbling, 30 minutes. Let sit 30 minutes before serving.