01 - Combine soy sauce, mirin, sake, sugar, sesame oil, minced garlic, grated ginger, toasted sesame seeds, and sliced green onion in a bowl. Whisk thoroughly until sugar dissolves completely.
02 - Add thinly sliced beef to the marinade, ensuring each piece is evenly coated. Let stand for at least 10 minutes at room temperature, or up to 1 hour in the refrigerator for deeper flavor penetration.
03 - Heat a grill pan or tabletop grill over high heat until properly hot. The surface should be searing hot to achieve proper caramelization.
04 - Place marinated beef slices on the hot grill. Cook for 1 to 2 minutes per side until just cooked through and lightly caramelized. Avoid overcooking to maintain tenderness.
05 - Grill sliced onion, bell pepper, mushrooms, and zucchini alongside the beef. Cook until edges are charred and vegetables are tender-crisp, approximately 3 to 4 minutes total.
06 - Arrange grilled beef and vegetables on serving plates. Serve immediately with steamed white rice while hot for the best dining experience.