Slice the flank steak (against the grain) into 1/4" thick bite-size slices
Cover the beef with thin dusting of cornstarch (to both sides)
Let the beef sit for about 10 minutes so that the cornstarch sticks.
Now, heat up one cup of oil in a wok or large skillet over medium heat
Add the beef to the oil and sauté for just 2-4 minutes (until the beef just begins to darken on the edges), stirring to ensure even cooking.
Use a large slotted spoon to take the meat out and onto paper towels, then pour the oil out of the wok or skillet & wipe clean
Put the pan back over the heat, dump the meat back into it, and simmer for one minute.
Add the sauce, cook for one minute while stirring
Then add all the green onions, cook for one more minute
Remove the beef and onions with tongs or a slotted spoon to a serving plate. (Leave the excess sauce behind in the pan)
Serve with the fried rice for an easy & yummy meal that is way better than take-out!