Preheat a very large cast-iron skillet over medium-high heat until it just begins to smoke.
While the pan is heating, slice the onions into 1/4-inch-thick rings and toss with 1 tablespoon of the oil, 1/4 teaspoon salt and a few grinds of pepper in a medium bowl. Add the onions to the skillet and cook, undisturbed, 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. Transfer to a plate.
Stir together the remaining 1 tablespoon oil and the fish sauce in the medium bowl that the onions were in. Add the radishes, watercress, cilantro 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.
Wipe out the skillet the onions cooked in and return it to medium-high heat. Sprinkle the steak with 3/4 teaspoon salt and a few grinds of pepper and put it into the hot skillet. Cook until well browned on one the bottom, 4 to 5 minutes; flip and cook until medium rare, 4 to 5 minutes more. Transfer to a cutting board and allow the steak to rest for a few minutes, then slice thinly against the grain.
Divide the salad mixture among 4 bowls. Top each bowl with slices of steak.
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