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1
Sprinkle 1 1/2 teaspoons salt and a few grinds of pepper over the steaks and set aside to come to room temperature.
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2
Meanwhile, put the garlic in a blender or food processor and pulse just to chop coarsely.
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3
Tear the parsley and cilantro into 2- to 3-inch lengths and add to the blender (tender stems and all), along with 1/4 cup of the olive oil, 2 tablespoons of the vinegar, and 1 teaspoon salt.
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4
Puree until smooth.
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5
Taste and add more salt or vinegar as needed.
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6
Heat 2 tablespoons of the remaining oil in a large skillet over medium-high heat.
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7
When the pan is smoking hot, add the steaks and cook undisturbed for at least 2 minutes.
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8
Check the browning and lower the heat if they seem to be browning too quickly.
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9
Cook, flipping occasionally, until the steaks reach the desired doneness, 4 to 5 minutes per side.
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10
(For medium-rare, an instant-read thermometer will read 125F at the thickest part of the steak.)
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11
Remove the steaks from the skillet, place on a cutting board, and cover loosely with foil.
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12
Let rest while you cook the onions.
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13
Add the remaining 1 tablespoon oil to the pan, along with the onion and a generous pinch of salt.
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14
Cook, stirring occasionally, until the onions have softened somewhat and are golden brown on the edges, 3 minutes.
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15
Add the remaining 2 tablespoons vinegar and cook, stirring, until the vinegar has evaporated and the onions are soft, 1 minute longer.
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16
Remove from the heat.
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17
Slice the steaks across the grain into 1/2-inch-thick strips, top with the sauteed onions, and serve the sauce alongside.
