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