Pat the flank steak dry with paper towels. Using a sharp knife, slice across the grain into very thin strips (julienne), about 1/4" wide and 2–3" long. Place the sliced steak in a shallow dish or resealable bag.

In a bowl, combine 1 cup soy sauce, 1/4 cup mirin, 2 tablespoons brown sugar, 1 tablespoon grated fresh ginger, 2 teaspoons minced garlic, 1 tablespoon rice vinegar, and 1 teaspoon sesame oil. Whisk until sugar dissolves.

Reserve 3 tablespoons of the marinade in a small saucepan for cooking (set aside). Pour the remaining marinade over the steak, press out excess air if using a bag, and refrigerate for 20–30 minutes (up to 2 hours).

About 10 minutes before cooking, remove the steak from the refrigerator to come closer to room temperature and allow the marinade to drain off slightly.

Make cornstarch slurry

In a small cup, stir 1 teaspoon cornstarch with 1 tablespoon cold water until smooth. This will be used to slightly thicken the reserved teriyaki sauce near the end.

Vegetables & Cooking

Heat a large skillet or cast-iron pan over medium-high heat until hot. Add 1 tablespoon neutral oil and swirl to coat.

Add the sliced mushrooms in a single layer. Let them sear undisturbed for 2 minutes to develop color, then stir and cook another 1–2 minutes until softened. Season lightly with salt and pepper. Push mushrooms to one side of the pan.

Add the marinated steak strips in a single layer across the hot pan (work in batches if necessary to avoid crowding). Spread them out so they contact the pan and cook without stirring for 1 minute to get a good sear.

Toss the steak briefly, then cook for another 1–2 minutes until just cooked through (thin strips cook quickly). Return any reserved juices from the steak to the pan.

Add the reserved 3 tablespoons of marinade to a small saucepan and bring to a gentle simmer over medium heat. Once simmering, whisk in the cornstarch slurry and cook for 30–60 seconds until slightly thickened into a glossy sauce. Remove from heat.

Pour the warmed teriyaki sauce over the cooked steak and mushrooms in the pan, toss to coat evenly, and cook together for 30–45 seconds so the sauce clings to the meat and vegetables.

Add the cut green onions to the pan and toss for 20–30 seconds just to wilt them slightly and warm through. Taste and adjust seasoning with salt and pepper as needed.

To Serve

If serving with rice, divide 2 cups steamed rice between plates or bowls. Spoon the teriyaki steak and mushroom mixture over the rice.

Garnish with 1 teaspoon toasted sesame seeds and additional sliced green onion if desired. Serve immediately with pickled ginger or cucumber on the side.