Roast and peel chiles. Wearing rubber gloves, cut chilies into 1/3-inch-thick strips. Halve onion lengthwise and cut each half lengthwise into 1/4-inch-thick slices.
In a heavy skillet cook onion in oil over moderately low heat, stirring frequently, until softened, about 5 minutes. Add chilies and salt, to taste, and cook, stirring, 5 minutes. Add creme fraiche or cream and cook, stirring, 2 minutes. Rajas may be made 1 day ahead and chilled, covered. Reheat rajas before serving.