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 sit for at least 10 minutes at room temperature, or up to 1 hour refrigerated for deeper flavor penetration.
03 - Heat a grill pan or tabletop grill over high heat until smoking hot. Lightly oil the grates to prevent sticking.
04 - Cook marinated beef slices for 1 to 2 minutes per side until just cooked through and edges are caramelized. Avoid overcooking to maintain tenderness.
05 - Grill sliced onion, bell pepper, shiitake mushrooms, and zucchini alongside the meat until charred at edges and tender-crisp, about 3 to 4 minutes total.
06 - Arrange grilled beef and vegetables on a platter. Serve immediately with steamed white rice and any remaining marinade on the side for dipping.