Marinate the Beef:
Place the thinly sliced flank steak in a bowl and add water, cornstarch, Shaoxing wine, sesame oil, oyster sauce, and baking soda. Stir well until the beef is evenly coated in the mixture. Allow it to marinate for at least one hour, or for enhanced flavor absorption, refrigerate overnight.
Prepare the Sauce:
In a mixing bowl or measuring cup, combine hot water, brown sugar, light soy sauce, oyster sauce, sesame oil, rice vinegar, and white pepper. Whisk thoroughly until all components are well incorporated. Set aside.
Dredge and Sear the Beef:
In a shallow dish, mix cornstarch and toasted sesame seeds. Lightly dredge each marinated beef slice in this mixture, ensuring an even coating. Shake off any excess.
Heat a wok or deep skillet over medium-high heat and add neutral oil. To check if the oil is ready, drop in a small piece of beef—it should sizzle immediately. Fry the beef slices in a single layer, allowing them to develop a crisp, golden crust for about one minute per side. Avoid overcrowding the pan, and fry in batches if needed. Once done, transfer to a plate in a single layer to preserve crispiness.
Finish the Dish:
Once all the beef is fried, let the oil settle before carefully draining it, leaving behind any residual particles. Wipe the wok clean and return it to medium heat, adding a small amount of fresh oil. Sauté the chopped garlic for about 15 seconds until fragrant.
Pour in the prepared sauce mixture, stirring to deglaze the pan and bring it to a simmer. Stir in the chopped scallions. Gradually add the cornstarch slurry while stirring continuously, allowing the sauce to thicken until it can coat the back of a spoon. If necessary, adjust the consistency by adding a small amount of hot water.
Return the crispy beef to the wok, tossing gently to ensure even coating with the sauce. Once combined, transfer to a serving plate and enjoy immediately with steamed rice.