How to Make It
Combine the arugula, sesame seeds, cheese, scallion, lemon juice, ½ teaspoon of the salt, ¼ teaspoon of the pepper, and ½ cup plus 2 tablespoons of the oil in a food processor. Pulse just to combine.
Heat a large cast-iron skillet over medium-high. Rub the steak with the remaining 1 tablespoon of oil and season with the remaining ½ teaspoon of salt and ¼ teaspoon of pepper. Cook, turning halfway through, until medium-rare, about 5 minutes total. Transfer to a cutting board; let rest for 5 minutes before thinly slicing against the grain.
Meanwhile, cook the pasta according to the package directions. Drain, reserving ¼ cup of the cooking water. Add the pasta, cooking water, and pesto to the pot and toss to coat.
Divide the pasta and steak among 4 plates and garnish with scallions, sesame seeds, and cheese.