Slice 250g of flank steak against the grain into ⅛in / 3mm slices. Mix with 1 teaspoon of soy sauce, 1 teaspoon of cornstarch, and 1 teaspoon of vegetable oil, and marinate for 10 minutes.
For the sauce, mix 2 teaspoons of cornstarch with ¼ cup of water until smooth. Stir in 2 tablespoons of soy sauce, ¼ cup of chicken broth, 1½ tablespoons of Shaoxing wine, and ¼ cup of brown sugar.
Toss the beef in ¼ cup of cornstarch to coat lightly. Heat ¼ cup of vegetable oil in a wok over medium-high heat.
Fry the beef in batches for about 45 seconds per side, until golden and crisp. Drain on paper towels.
Pour off all but about 1 tablespoon of the oil. Fry ½ teaspoon of minced ginger and 2 crushed garlic cloves for about 15 seconds. Add the sauce and simmer for about 1½ minutes, until glossy and sticky.
Return the beef with 2 scallions, cut into 1½in / 4cm pieces, and toss for 30 seconds.
