Heat ½ cup oil in a wok over medium high heat.
While oil is heating in the wok, add ¼ cup corn starch to the marinated beef. Use your fingers to lightly coat the beef strips.
Once oil is hot, add several strips of beef and cook the first side for 45-60 seconds or until golden and crisp.
Flip beef over and cook on the other side for 30-45 seconds until golden and crisp.
Remove cooked beef to a paper towel lined plate.
Repeat with the remaining beef. Add more oil if needed.
Once all the beef strips have been cooked, discard all but one tablespoon of oil from the wok.
Add ginger, garlic, white part of the scallions and sliced red chili (if using) to the wok, stir fry for 15 seconds. Watch it carefully, do not allow garlic to burn.
Add the sauce to the wok. Bring the sauce to a simmer and cook for 1-2 minutes until sauce thickens. Add water to thin the sauce if needed.
Return the cooked beef to the sauce in the wok and toss to coat with the sauce for about 30 seconds.
Serve over cooked white rice and a generous sprinkle of the sliced green tops from the scallions.