Lay chiles on their sides on racks of gas burners, 1 or 2 chiles per burner, then turn flames on moderately high.
Alternatively, put chiles on rack of a broiler pan 2 inches from heat.
Roast chiles, turning frequently with tongs, until skins are blistered and lightly charred all over, 4 to 6 minutes (6 to 8 minutes if broiling). Transfer chiles immediately to a large bowl, then cover with a plate. Steam 20 minutes, then peel or rub off skin.
