Step 1

Heat a 12-inch cast-iron skillet over medium-high. Add 1 tablespoon of the oil to skillet, and swirl to coat. Sprinkle both sides of steak evenly with 1½ teaspoons of the salt and 1 teaspoon of the pepper. Cook steak in hot skillet, flipping about every 2 minutes, until a thermometer inserted in thickest portion of meat registers 125°F (for medium-rare), 5 to 7 minutes. Transfer steak to a cutting board; cover loosely with aluminum foil, and let rest 10 minutes.

Step 2

While steak rests, pulse parsley, cilantro, shallot, vinegar, garlic, and remaining ½ teaspoon each salt and pepper in a food processor until finely chopped, about 10 pulses, stopping to scrape down sides as needed. With processor running, slowly pour remaining ½ cup oil through food chute until mixture is thoroughly combined but still chunky, about 10 seconds.

Step 3

Cut steak diagonally against the grain into ¼-inch-thick slices, and arrange on a platter. Serve with roasted potatoes and herb sauce.