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Preheat the oven to 375°. Rub the cut sides of a garlic clove on the bottom and sides of a 9 x 13-inch casserole.
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Peel and slice the potatoes, placing the potatoes immediately into large bowl of ice water as you work to prevent browning.
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Prepare the peeled, seeded green chiles by slicing lengthwise and opening up flat into a single layer.
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Grate the cheese and place the cream in a large saucepan on stove top over a medium heat and bring to a simmer.
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Drain and pat dry the potatoes in a dishtowel then add into the saucepan of simmering cream for 5 minutes.
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Using a slotted spoon, remove the potato slices placing half in the baking dish to create the first layer. Sprinkle with a pinch of salt and pepper. If using the green chiles, arrange flat in a single layer over the potatoes.
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Cover the chiles with the remaining potatoes, pressing down to smooth evenly. Pour the remaining cream from the saucepan over the potatoes, distributing evenly if making individual gratins, top with the grated Gruyere cheese and another sprinkle of salt and pepper.
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Place the gratin on a baking sheet and bake in a 375° oven for about 45 minutes, or until the potatoes are tender and the top is browned and bubbly.
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Remove from the oven and allow to rest about 10 minutes before serving. Sprinkle the top with the parsley and serve.