Whisk together soy sauce, sugar, oil, garlic, and kiwi (if using) in a medium bowl.
Holding knife at a 45-degree angle to cutting board, slice steak as thinly as possible against the grain. Add steak to bowl and toss well to coat. Cover and refrigerate for 1 to 1 ½ hours.
Drain excess marinade from steak mixture. Heat 1 ½ teaspoons oil in a large cast-iron skillet over medium-high. Add ¼ of steak to pan, spreading slices out flat as much as possible. Cook until lightly charred, turning occasionally, 2 to 3 minutes. Place cooked steak in a serving bowl. Repeat process in 3 more batches with remaining oil and steak. Garnish with toasted sesame seeds and scallions.
