Preheat a very large cast-iron skillet over medium-high heat until it just begins to smoke, up to 5 minutes. Toss the onions in a medium bowl with 1 tablespoon of the sesame oil and some salt and pepper. Add the onions to the skillet and cook, untouched, until they char slightly, about 2 minutes. Stir occasionally with a wooden spoon until they are further charred and slightly softened but still crunchy, 3 to 4 minutes more.
Stir together the remaining 1 tablespoon sesame oil and the fish sauce in a medium bowl. Add the radishes and cooked onions and toss until well coated. Allow the mixture to sit at least 5 minutes, so the juices from the onion combine with the dressing. Fold in the cilantro.
Copyright 2013 Cooking Channel, LLC. All rights reserved.