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1
Preheat a very large cast-iron skillet over medium-high heat until it just begins to smoke.
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2
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.
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3
Add the onions to the skillet and cook, undisturbed, until they char slightly, about 2 minutes.
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4
Stir occasionally with a wooden spoon until they are further charred and slightly softened but still crunchy, 3 to 4 minutes more.
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5
Transfer to a plate.
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6
Stir together the remaining 1 tablespoon oil and the fish sauce in the medium bowl that the onions were in.
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7
Add the radishes, watercress, cilantro and cooked onions and toss until well coated.
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8
Allow the mixture to sit at least 5 minutes, so the juices from the onion combine with the dressing.
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9
Wipe out the skillet the onions cooked in and return it to medium-high heat.
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10
Sprinkle the steak with 3/4 teaspoon salt and a few grinds of pepper and put it into the hot skillet.
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11
Cook until well browned on one the bottom, 4 to 5 minutes; flip and cook until medium rare, 4 to 5 minutes more.
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12
Transfer to a cutting board and allow the steak to rest for a few minutes, then slice thinly against the grain.
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13
Divide the salad mixture among 4 bowls.
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14
Top each bowl with slices of steak.
