FOR THE HERB PASTE:
Pulse cilantro, garlic, scallions, jalapeño, and cumin in food processor until finely chopped, ten to twelve 1-second pulses, scraping down sides as necessary.
Add oil in a thin stream through food processor tube and process until mixture is smooth and resembles pesto, about 15 seconds, scraping down sides of workbowl as necessary.
Transfer 2 tablespoons herb paste to medium bowl; whisk in lime juice and set aside (this will be used to mix with the steak after resting/slicing).
FOR THE STEAK:
Using dinner fork, poke each piece of steak 10 to 12 times on each side.
Place in large glass baking dish; rub all sides of steak pieces evenly with salt and then coat with remaining herb paste.
Cover with plastic wrap and refrigerate at least 30 minutes or up to 1 hour.
Scrape herb paste off steak and sprinkle all sides of pieces evenly with sugar and pepper.
Heat oil in 12-inch heavy-bottomed nonstick skillet over medium-high heat until smoking.
Place steak in skillet and cook until well browned, about 3 minutes.
Flip steak and sear until second side is well browned, 2 to 3 minutes.
Using tongs, stand each piece on a cut side and cook, turning as necessary, until all cut sides are well browned and internal temperature registers 125 to 130 degrees on an instant-read thermometer, 2 to 7 minutes.
Transfer steak to cutting board and let rest 5 minutes.
FOR THE TACOS:
Using sharp chef’s knife or carving knife, slice steak pieces across grain into 1/8-inch-thick pieces.
Transfer sliced steak to bowl with herb paste-lime juice mixture and toss to coat.
Season with salt.
Spoon small amount of sliced steak into center of each warm tortilla and serve immediately, passing toppings separately.