INGREDIENTS
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5 garlic cloves, divided
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1 tablespoon unsalted butter, room temperature
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2 medium shallots, quartered through root ends
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2 1/2 cups heavy cream
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1 tablespoon kosher salt
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1 teaspoon freshly ground black pepper
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1 tablespoon thyme leaves, plus more
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4 pounds russet potatoes, scrubbed, very thinly sliced on a mandoline
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3 ounces Gruyère, finely grated
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1 ounce Parmesan, finely grated