First, place 1 cup of cornstarch in a large bowl. Toss the flank steak in cornstarch. Mix until everything is coated evenly.
Then pour the oil into a wok. Heat it up over high heat until it reaches 350°F. You can test the oil with 1 steak strip first. If you see lots of bubbles around it then the oil is ready.
Put ¼ of the beef strips work. Quickly separate them with chopsticks or a fork. Deep fry until the coating is crisp and golden, about 3 minutes. Remove the beef and drain on a paper towel lined plate. Repeat with the remaining beef strips.
To make the sauce, in a bowl, whisk together the soy sauce, rice vinegar, rice wine, sugar, sesame oil, and red pepper flakes.
In another pan, add 2 tablespoons of oil, the carrot, scallions (the white and middle parts), ginger, and garlic. Cook over high heat until the aroma comes out.
Pour in the sauce and cook until it’s boiling. Introduce the cornstarch slurry to thickening the sauce.
Transfer the beef strips back into the pan. Stir-fry until combined. Lastly, add the green part of the scallions.