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, tossing pieces to ensure even coating. Let stand at room temperature for at least 10 minutes, or refrigerate up to 1 hour for deeper flavor penetration.
03 - Preheat grill pan or tabletop grill over high heat until smoking hot. Lightly oil the surface to prevent sticking.
04 - Place marinated beef slices on the hot grill. Cook for 1 to 2 minutes per side until just cooked through and edges show slight caramelization. Avoid overcooking to maintain tenderness.
05 - Arrange onion, bell pepper, mushrooms, and zucchini slices on the grill alongside beef. Cook until vegetables develop char marks and become tender-crisp, approximately 3 to 4 minutes.
06 - Transfer grilled beef and vegetables to a serving platter. Accompany with steamed white rice. Serve immediately while hot.