Make the herb paste first — the steak needs 30–60 minutes in it. Reserve 2 tablespoons of the paste before the steak touches it; that becomes the dressing. Cook to 125°F, rest 5 minutes, slice across the grain.
Herb paste
Steak
To serve
Combine the cilantro, scallions, garlic, jalapeño, cumin, and 4 teaspoons of canola oil. Blend and purée into a paste.
Transfer 2 tablespoons of the paste to a small bowl and whisk it with the lime juice. Set aside — this is the finishing dressing.
Slice the steak with the grain into equal strips 1½–2 inches wide. Using a dinner fork, poke each piece 10–12 times on each side. Coat each slice thoroughly with the remaining herb paste. Cover with plastic wrap and refrigerate 30–60 minutes.
Scrape the herb paste off the steak. Sprinkle all sides evenly with sugar and season with pepper.
Heat 2 teaspoons of oil in a nonstick skillet over medium-high heat until just smoking. Brown the pieces well on all sides and cook until the meat reaches 125°F, about 7–14 minutes. Let the meat rest 5 minutes.
Warm the tortillas, finely chop the onion, and slice the lime into wedges.
Slice the steak across the grain and transfer to a medium bowl. Toss with the reserved herb-lime mixture, and season with salt and pepper to taste.