01 - Whisk together soy sauce, mirin, sake, sugar, sesame oil, minced garlic, grated ginger, sesame seeds, and sliced green onion in a mixing bowl until sugar dissolves completely.
02 - Add thinly sliced beef to the marinade, toss thoroughly to coat all pieces, and let stand at room temperature for at least 10 minutes to absorb flavors.
03 - Heat a grill pan or tabletop grill over high heat until properly hot, ensuring good searing capability for the beef and vegetables.
04 - Place marinated beef slices on the hot grill and cook for 1–2 minutes per side until just cooked through and lightly caramelized, being careful not to overcook.
05 - Arrange sliced onions, bell peppers, shiitake mushrooms, and zucchini on the grill alongside the meat, cooking until charred at edges and tender-crisp.
06 - Transfer grilled beef and vegetables to a serving platter and immediately serve hot alongside steamed white rice while everything remains warm.