Active Time

　　45 min

　　Total Time

　　2 1/2 hr

　　In Mexican cuisine, rajas refers to thin strips of roasted chiles. Although they commonly spice up everything from stews to tamales, rajas are best when adding a kick to creamy dishes. Here, forest-green poblanos lend a mild, almost fruity heat to a potato gratin.

　　

Ingredients

Makes 8 (side dish) servings

　　1 1/2 pounds fresh poblano chiles (about 5)

　　1 pound onions, cut lengthwise into 1/4-inch strips

　　1 tablespoon vegetable oil

　　3 pounds large Yukon Gold potatoes

　　1 1/2 cups heavy cream

　　3/4 cup whole milk

　　Equipment: an adjustable-blade slicer

　　

Roast chiles and make rajas:

Step 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.

　　

Step 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.

　　

Step 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 rajas from heat. Reserve 1/2 cup rajas for topping.

　　

Make gratin:

Step 4

Preheat oven to 400°F with rack in middle. Generously butter a 3-quart shallow baking dish.

　　

Step 5

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 rajas, then pour mixture evenly into baking dish. Sprinkle reserved 1/2 cup rajas on top.

　　

Step 6

Bake until potatoes are tender, 45 minutes to 1 hour. Let stand 15 minutes before serving.

　　Cooks’ notes:

　　•Chiles can be roasted by broiling on a broiler pan 2 inches from heat, turning, 8 to 10 minutes.

　　•Rajas can be made 3 days ahead and chilled.

　　•Gratin can be made 1 day ahead and chilled. Bring to room temperature and reheat, covered, in a 350°F oven (about 30 minutes).