Bring beef to room temperature. Mix soy sauce, sugar, mirin, sesame oil, garlic, spring onion, sesame seeds, and black pepper in a bowl. Add beef and stir to coat. Cover and refrigerate for at least 1 hour.
Heat a grill or broiler to high heat. Cook beef for roughly 2 minutes per side, turning frequently, until cooked through. If using a broiler, place marinated beef on a tray before cooking.