Poblano Potato Gratin

Directions

1. 1
   Roast chiles on their sides on racks of gas burners on high, turning with tongs, until skins are blackened all over, about 10 minutes. Immediately transfer to a bowl and let stand, covered tightly, 10 minutes.
2. 2
   When chiles are cool enough to handle, peel or rub off skin. Slit chiles lengthwise, then stem, seed, and devein. Cut lengthwise into thin strips.
3. 3
   Cook onions with 1 teaspoon salt in oil in a 12-inch heavy skillet over medium-low heat, stirring occasionally, until golden, about 8 minutes. Stir in chiles and remove
4. 4
   from heat. Reserve 1/2 cup
5. 5
   for topping.
6. 6
   Preheat oven to 400°F with rack in middle. Generously butter a 3-quart shallow baking dish.
7. 7
   Peel potatoes, then cut crosswise into 1/16-inch-thick slices with slicer. Transfer to a small heavy pot. Add cream, milk, and 1 teaspoon salt and bring just to a boil over medium heat, stirring occasionally (liquid will thicken). Stir in
8. 8
   , then pour mixture evenly into baking dish. Sprinkle reserved 1/2 cup
9. 9
   on top.
10. 10
    Bake until potatoes are tender, 45 minutes to 1 hour. Let stand 15 minutes before serving.