Slice the Flank Steak: Use a sharp knife to cut the flank steak in half lengthwise, then slice each piece crosswise against the grain into ¼-inch-thick slices.

Marinate the Beef: Place the sliced steak in a medium bowl, add the cornstarch, baking soda, soy sauce, and vegetable oil, and toss until coated. Let it marinate for 5 minutes.

Prepare the Sauce: Whisk together water, soy sauce, brown sugar, mirin, and white pepper (if using) in a medium bowl and set aside.

Cook the Beef: Heat 2 tablespoons of oil in a wok or large skillet over high heat. Spread the beef in an even layer and let it cook undisturbed until crispy and browned, about 30 to 60 seconds per side. Transfer the cooked beef to a bowl, leaving the oil in the wok. Repeat with a second batch if needed.

Sauté Aromatics: Lower the heat to medium-low and add the remaining 1 tablespoon of oil. Add the ginger, garlic, and red chili flakes (if using), and stir for about 1 minute, being careful not to burn the ginger.

Combine Everything: Add the scallions and the cooked beef (with any juices) to the wok and stir well.

Thicken the Sauce: Whisk the sauce again, pour it into the wok, and stir until it thickens, about 2 minutes.

Serve: Transfer to a platter and serve with steamed rice.