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.