01 - Whisk together soy sauce, mirin, sake, sugar, sesame oil, minced garlic, grated ginger, toasted sesame seeds, and sliced green onion in a mixing bowl until sugar dissolves completely.
02 - Add thinly sliced beef to the marinade, tossing thoroughly to coat each piece. Let stand at room temperature for at least 10 minutes, or refrigerate up to 1 hour for deeper flavor penetration.
03 - Heat a grill pan or tabletop grill over high heat until smoking hot. The surface must be sufficiently hot to achieve proper searing and caramelization.
04 - Place marinated beef slices on the hot grill. Cook for 1 to 2 minutes per side until just cooked through and edges develop a slight caramelized char. Avoid overcooking to maintain tenderness.
05 - Grill sliced onions, bell peppers, shiitake mushrooms, and zucchini alongside the meat. Cook until vegetables develop char marks and become tender-crisp, approximately 3 to 4 minutes total.
06 - Arrange grilled beef and vegetables on serving plates. Accompany with steamed white rice. Serve immediately while hot for optimal texture and flavor.