Poblano Potato Gratin
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 … Continued