Arrange meat in a single layer on a large cutting board. Cover with plastic wrap and pound with a meat mallet (or the bottom of a small skillet) until meat is super thin, about 1/16-inch thick.
In a large bowl, combine soy sauce, scallions, sugar, garlic, pepper and sesame oil. Add meat and toss to evenly coat, massaging sauce into meat.
Step 2 : Preparing the cooking
In a large skillet, heat 1/4 cup of the safflower oil over medium. Add onion and shishito peppers, and season with salt and pepper. Cook, stirring occasionally, until tender and lightly caramelized, 8 to 10 minutes. Divide among the rolls.
In the same skillet, heat 2 tablespoons safflower oil over medium. Add half of the beef and season with salt and pepper. Cook, stirring to break up the meat, until golden brown and cooked through, about 3 minutes.
Step 3 : To serve
Form into 2 even mounds and top each mound with 2 slices of cheese. Remove from heat and let stand until cheese melts, about 1 minute. Using a flat spatula or large serving spoon, scoop into 2 rolls and drizzle with pan juices. Wipe out skillet with paper towels. Repeat with remaining safflower oil, beef and cheese to make 2 more sandwiches.