Split the flank steak along the grain into two long pieces, then slice each piece against the grain into 1/8” (3 mm) thin pieces. Transfer to a medium bowl. Add the light soy sauce, baking soda and white pepper. Mix thoroughly with your hands to evenly coat the beef. Add the cornstarch and mix again. If the cornstarch does not distribute evenly, add 1 to 2 teaspoons of water and mix again. Set aside to marinate for 15 minutes while preparing the other ingredients.