1 tablespoon Light Brown Sugar
2 teaspoons Ground Pepper
¾ teaspoon Kosher Salt, plus a pinch, divided
1 pound Flank Steak, trimmed
1 pound Shishito, peppers
1 tablespoon Sesame Oil , plus 1 teaspoon, divided
1 teaspoon Rice Vinegar
1 Scallion, minced
Position rack in upper third of oven; preheat broiler to high. Line a rimmed baking sheet with foil.
Combine brown sugar, pepper and 3/4 teaspoon salt in a small bowl. Pat steak dry, then rub the brown sugar mixture on both sides. Place on the prepared pan.
Toss peppers with 1 tablespoon oil in a large bowl. Scatter the peppers around the steak.
Broil, flipping the steak and stirring the peppers once halfway through, until an instant-read thermometer inserted in the thickest part of the steak registers 145 degrees F for medium rare and the peppers are tender, 10 to 15 minutes.
Meanwhile, whisk vinegar, scallion and the remaining 1 teaspoon oil and pinch of salt in the large bowl.
Transfer the steak to a clean cutting board and let rest for 5 minutes. Add the peppers and any accumulated juices from the steak back to the bowl and toss to coat. Slice the steak against the grain and serve with the peppers.