PREPARE THE MARINADE: In a bowl, stir together the soy sauce, onion, sugar, mirin, garlic, sesame oil and black pepper. If you have the time, refrigerate the marinade for a day on its own. Place the beef, scallions and sesame seeds into a large zip-top bag; pour the marinade on top. Compress the bag to remove excess air, then refrigerate for at least 2 hours, preferably 24 hours.
TO PAN-SEAR THE MEAT: Remove the meat from the marinade and pat dry. Preheat a large cast-iron skillet or grill pan over high heat, until smoking hot. Using a towel, rub on some vegetable oil. Working in batches, grill the marinated beef until cooked through, about 5 minutes, turning frequently until the meat begins to caramelize. Work in batches, serving the meat as it ready.
OR TO GRILL THE MEAT: Line the grill grates with foil. Heat on high until you can hold your hand 6 inches over the grate for only 2 to 3 seconds. Grill the marinated beef, uncovered, until cooked through, about 5 minutes, turning frequently until the meat begins to caramelize. Work in batches, serving the meat as it is ready.
Serve the beef with the rice, lettuce, perilla, kimchi and Ssamjang. To eat, make a lettuce wrap (called ssambap): take a lettuce or perilla leaf and pack on a couple spoons of rice. Layer with kimchi and some bulgogi and top with a dab of Ssamjang.
