Combine the sugar, soy sauce, sake, garlic, scallions, ginger, and sesame oil in a medium-size flat dish or a zip-top bag, stirring to make sure the sugar is dissolved. Cover the steaks with plastic wrap and gently pound them 1/4 inch thick with the flat side of a meat mallet. Add them to the marinade and let them sit for 20 minutes, covered, turning them once after 10 minutes. Coat a large skillet with the vegetable oil and set it over high heat until the oil is just barely smoking. Sprinkle both sides of the meat with salt and pepper, add the meat to the pan, lower the heat slightly, and sear the steaks (in batches if necessary) for 2 to 3 minutes. on each side for medium rare. The steaks should be very rosy pink. Transfer them to a cutting board, tent with foil, and let rest for 3 minutes. Cut the steaks into 1/4-inch-thick slices and serve.