Cut the flank steak in half making roughly two squares of meat. Place one between two pieces of plastic wrap and pound with a meat pounder or rolling pin until it is about ¼-inch thick. Repeat with the other piece. Sprinkle one side of each piece of meat with some salt and pepper.
Trim the scallions to fit inside of the meat going the long way. Place 4 scallions on each piece of meat, placing two of the white parts at both ends. Roll the meat around the scallions. Secure with toothpicks or tie with string. Brush with 2 tablespoons of the Teriyaki Sauce.
Heat the oil in a skillet over medium-high heat. Brown the steak rolls on all sides, about 1 minute per side. Once browned all over, reduce the heat to low, cover the pan, and cook for 10 minutes, turning once. Remove from the pan and let sit for 5 minutes before slicing. Turn the heat up to medium-high, add the remaining 4 tablespoons of Teriyaki Sauce and cook, stirring, until it thickens into a glaze, 2-3 minutes.
Remove the toothpicks or string and slice the rolls about ½-inch thick. Brush with the glaze and serve.