In a bowl mix together the sliced beef, oil, soy sauce, and cornstarch. Marinate for 30 minutes.
In a small bowl, mix brown sugar, hot water, soy sauces and sesame oil.
In another small bowl mix the cornstarch and water slurry for the sauce.
Heat 1/3 cup vegetable oil in the wok over high heat. Just before the oil starts to smoke, dredge the marinated beef slices in 1/3 cup of cornstarch until all pieces are evenly coated.
Transfer the flank steak to the wok, and sear for 1 to 2 minutes. Flip the pieces over and sear the other side for 1 to 2 minutes. Remove beef to a bowl.
Turn the heat to medium-high. Add the garlic, ginger and dried chili peppers. After about 15 seconds, add the sauce.
Let the sauce simmer for about 2 minutes and slowly stir in the cornstarch slurry mixture. Cook until the sauce has thickened enough to coat the back of a spoon. If too thick, add tablespoon or two of water.
Add the beef and scallions to the wok and toss everything together for another 30 seconds. Serve with steamed rice.
Video
Notes
Slice the flank steak into 3 long pieces cutting with the grain. Then slice each piece against the grain into 1/4-inch thick pieces (as shown). If preferred, slice into 1/4-inch pieces on the diagonal.