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1
Preheat oven to 375 degrees F.
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2
Lay chiles on their sides on rack of a gas burner and turn flame on high.
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3
(Or put chiles on rack of broiler pan about 2 inches from heat.)
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4
Roast chiles, turning with tongs, until skins are blackened, 5 to 8 minutes.
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5
Transfer chiles to a bowl and let stand, covered, 15 minutes.
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6
Rub skins off chiles with paper towels and discard stems and seeds (if desired, devein to further reduce heat).
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7
Cut the chiles into 1/4-inch-thick strips.
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8
Bring cream with garlic just to a simmer and remove from heat.
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9
Peel potatoes and cut crosswise into 1/16-inch-thick slices using mandoline.
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10
Arrange one-third of potatoes evenly in bottom of a well-buttered 1 1/2-quart shallow baking dish, overlapping them slightly, and sprinkle with salt, to taste, and 1/2 of chiles.
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11
Repeat, ending with a layer of potatoes.
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12
Remove garlic from cream and pour cream over potatoes.
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13
Cover dish with foil and bake in lower third of oven 45 minutes.
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14
Remove foil and bake, basting the top occasionally, until gratin is golden brown on top and bubbling, about 30 minutes more.
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15
Cool slightly before serving.
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16
Cooks' note: Gratin can be baked 1 day ahead, then cooled completely before being chilled, covered.
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17
Bring to room temperature before reheating, covered, in a preheated 350 degree F oven.
