01 - Combine soy sauce, mirin, sake, sugar, sesame oil, garlic, ginger, sesame seeds, and green onion in a mixing bowl. Whisk thoroughly until sugar dissolves completely.
02 - Add sliced beef to the marinade, ensuring each piece is coated evenly. Let stand for at least 10 minutes at room temperature, or up to 1 hour in refrigerator for deeper flavor penetration.
03 - Preheat grill pan or tabletop grill over high heat until surface is smoking hot. Proper temperature ensures quick searing and caramelization.
04 - Place marinated beef slices on hot grill. Cook for 1–2 minutes per side until just cooked through and edges develop a slight char. Avoid overcooking to maintain tenderness.
05 - Arrange onion, bell pepper, mushrooms, and zucchini on grill. Cook alongside meat until vegetables are tender-crisp with light char marks, approximately 3–4 minutes.
06 - Transfer grilled beef and vegetables to serving platter. Accompany with steamed white rice. Serve immediately while hot for optimal texture and flavor.