01 - Combine soy sauce, mirin, sake, sugar, sesame oil, minced garlic, grated ginger, sesame seeds, and sliced green onion in a mixing bowl. Whisk thoroughly until sugar dissolves completely.
02 - Add thinly sliced beef to the marinade, ensuring each piece is thoroughly coated. Let stand for at least 10 minutes, or up to 1 hour for deeper flavor penetration.
03 - Heat a grill pan or tabletop grill over high heat until properly searing temperature is reached. The surface should be smoking hot for optimal caramelization.
04 - Place marinated beef slices on the 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 - Grill sliced onions, bell peppers, shiitake mushrooms, and zucchini alongside the meat. Cook until tender-crisp with visible char marks, approximately 3-4 minutes total.
06 - Arrange grilled beef and vegetables on serving plates. Serve immediately alongside steamed white rice while hot.