Heat 2 teaspoons of vegetable oil in a medium saucepan over med-low heat. Don't get the oil too hot.

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

Dissolve the brown sugar in the sauce, then raise the heat to medium and boil the sauce until the sauce thickens, about 2-3 minutes. Remove the saucepan from the heat.

Slice the flank steak against the grain into 1/4-inch thick bite-size 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.

Dip the steak pieces into the cornstarch and apply a very thin dusting to both sides of each piece of beef. Let the beef sit for 10 minutes so that the cornstarch sticks.

Meanwhile, heat up one cup of oil over medium heat in a wok or a skillet in which the beef can be mostly covered with oil. Heat the oil until it's nice and hot, but not smoking.

Add the beef to the oil and sauté until the beef just begins to darken on the edges, about 2 minutes. Stir to cook evenly. The beef does not have to be fully cooked here since it will be cooked further later.

Use a slotted spoon to take the meat out and onto paper towels, then pour the oil out of the wok or skillet.

Place the pan back over medium heat, put the beef pieces back into it and simmer for 1 minute. Add the sauce, cook for 1 minute while stirring, then add all the green onions.

Sprinkle white sesame, cook for 1 more minute, then remove the beef and onions with tongs or a slotted spoon to a serving plate. Leave the excess sauce behind in the pan.