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1
Heat the oven to 350 degrees, and place rack in the center.
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2
Bring a large pot of lightly salted water to a boil; set a bowl of ice water on the side.
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3
Boil the chard leaves until tender, 3 to 5 minutes, then transfer with a slotted spoon to the ice water.
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4
Squeeze them dry and chop roughly.
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5
Boil the diced stems until tender, 3 to 5 minutes.
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6
Drain well and add to chopped chard leaves.
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7
In a small saucepan, bring the heavy cream, garlic, shallot, thyme and bay leaf to a simmer.
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8
Cook until reduced by half, about 25 minutes.
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9
Strain out the solids and add the nutmeg.
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10
Meanwhile, slice the potatoes into 1/8-inch-thick rounds with a mandoline or sharp knife.
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11
Butter a 12- to 14-inch gratin dish.
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12
Assemble the gratin by layering the ingredients in this order: a single, slightly overlapping layer of one-third of the potato slices, a sprinkling of salt and pepper, one-third of the Gruyere, half the Swiss chard and one-third of the reduced cream.
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13
Repeat once, and then top with one more layer of potato, salt and pepper, and the rest of the Gruyere and cream.
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14
Bake until the top is browned and the potatoes are fork-tender, about 45 minutes.
