Heat vegetable oil in a medium saucepan over medium heat. Don't get the oil too hot.

Add ginger and garlic to the pan. Then quickly add the soy sauce and water before the garlic scorches.

Dissolve the brown sugar in the sauce, then increase the heat to medium and boil the sauce for 2-3 minutes or until the sauce thickens.

Remove from heat.

Slice the flank steak against the grain into 1/4" thick slices. Tilt the blade of your knife at about a forty five degree angle to the top of the steak so that you get wider cuts.

Place the cut steak in a large resealable bag and add cornstarch. Shake the bag to coat the steak completely in cornstarch and let sit for 10 minutes.

As the beef sits, heat up the vegetable oil in a skillet over medium heat. You want it hot but not smoking.

Add the beef to the oil and sauté until brown. Stir around a little so that it cooks evenly.

After a couple minutes, use a large slotted spoon or tongs to take meat out and place on paper towels. Dab excess oil off meat with a paper towel and add to medium saucepan with the sauce in it.

Put the pan back on the oven over medium/low heat.

Serve over cooked rice.

Garnish with green onion, if desired.