Pulse the cilantro, scallions, garlic, jalapeño, and cumin together in a food processor until finely chopped, 10 to 12 pulses, scraping down the sides of the bowl as needed. Add the oil and process until the mixture is smooth and resembles pesto, about 15 seconds, again scraping down the sides of the bowl as necessary. Scoop 2 tablespoons of the herb paste into a large bowl, stir in the lime juice, and forget about it until just before serving.
Using a fork, poke each piece of steak 10 to 12 times on each side. Place the steak in a large baking dish, rub all sides of the steak evenly with 1 1/2 teaspoons salt, and then coat with the cilantro paste. Wrap the steaks with plastic wrap and refrigerate for 30 to 60 minutes.
Scrape the cilantro paste off the steak and sprinkle all sides evenly with the sugar and 1/2 teaspoon black pepper. Heat the oil in 12-inch nonstick skillet over medium-high heat until just smoking. Carefully place the steak in the skillet and cook until well browned on the first side, 3 to 5 minutes. Flip the steak and sear until well browned on the second side, 3 to 5 minutes.
Using tongs, hold each piece of steak so a cut side is against the skillet and continue cook, turning as necessary to brown all sides well, to the desired doneness. [Editor's Note: We strongly encourage you to cook this only until medium-rare as flank steak tends to become really quite tough when cooked to medium or well done.] Place the steak on a carving board, loosely tent it with foil, and let rest for at least 5 minutes.