Reserve 3 tablespoons of the marinade in a small saucepan for reducing later. Add the cornstarch to the reserved marinade and stir in 1 tablespoon of water to make a thin slurry; set aside.

Place the thinly sliced steak in a shallow dish or resealable plastic bag. Pour the remaining marinade over the steak, add freshly ground black pepper, and turn to coat. Marinate in the refrigerator for 20–30 minutes (or up to 2 hours for more flavor), turning once halfway through.

Mushrooms & Vegetables

While the steak marinates, clean and halve or slice the mushrooms. Trim the scallions and cut into 1–1.5-inch pieces, keeping some white and green parts separated if you want different cook times.

Heat a large skillet or cast-iron pan over medium-high heat. Add 1 teaspoon vegetable oil. Add mushrooms in a single layer; season with 1/4 teaspoon salt. Sear without stirring for 1–2 minutes to develop color, then stir and cook an additional 2–3 minutes until tender and browned. Stir in 1/2 teaspoon sesame oil (optional) and remove mushrooms to a bowl.

In the same pan, if needed, add a touch more oil and quickly sauté the scallion whites for about 30–45 seconds until slightly softened; reserve the scallion greens for garnish. Remove to the bowl with mushrooms and keep warm.

Cook & Finish

Preheat a grill or set the skillet to medium-high/high heat. If grilling, oil the grate lightly. Remove steak from marinade, letting excess drip off. Grill or sear steak slices in batches in a hot pan or on the grill for 30–60 seconds per side (thin slices cook very quickly) until just browned and medium-rare to medium, about 1–2 minutes total per batch depending on thickness.

As each batch finishes, transfer steak to the bowl with mushrooms and scallions (or a warm platter). Do not overcrowd the pan so slices sear rather than steam.

Place the reserved 3 tablespoons of marinade with cornstarch slurry in a small saucepan over medium heat. Bring to a gentle simmer, stirring constantly until sauce thickens and becomes glossy, about 1–2 minutes. If too thick, add 1–2 teaspoons water to reach desired consistency. Taste and adjust: add 1/2 teaspoon sugar or a pinch of salt if needed.

Pour the warmed teriyaki glaze over the cooked steak, mushrooms, and scallions and toss briefly to coat. Heat together for 30–60 seconds to marry flavors, then remove from heat.

Garnish with reserved scallion greens and toasted sesame seeds. Serve immediately over steamed rice or noodles, and add fresh cilantro if using.