PREPARE IN ADVANCE: Cut onions in half lengthwise, the cut into 4" pieces. Place the steak between sheets of greasproof paper and pound into 1/8" thickness. Cut the steaks in half crosswise. Arrange the strip of onions down the length of each piece of meat. Starting with the wide end, roll into tight cylinders. Secure with toothpicks. TO COOK: Preheat grill. Dip rolls in Teriaki sauce and grill them 3" from heat for 3 minutes. Dip again in sauce and grill for another 3 minutes. Remove toothpicks, trim ends, and cut into 1" pieces. Stand each piece on end.