In a large bowl, combine the flank steak and the cornstarch, tossing so the flank steak is well coated. Set aside.
1 pound flank steak, 1/4 cup cornstarch
In a small bowl, whisk together the water, soy sauce, brown sugar, and cornstarch. Set aside.
1/3 cup water, 1/4 cup low sodium soy sauce, 1/2 cup brown sugar, 2 teaspoons cornstarch
In a large skillet over medium heat, heat the oil. Once it is shimmering, cook the meat in batches, being careful to not over crowd the pan. Cook for about 3 to 5 minutes. Remove the beef and transfer it to a plate, and tent with foil. Drain off all but 1 tablespoon of the oil from the pan.
1/4 cup vegetable oil
Add the garlic, ginger, and the whites of the green onions, sauté until just fragrant being careful not to burn the garlic.
2 garlic cloves, 1 tablespoon minced ginger, 4 green onions
Add the sauce to the skillet, scraping up any browned bits. Bring to a simmer and cook until the sauce has thickened, about 1 minute. Then return the beef to the skillet, tossing to combine. Garnish with the remaining green onions and serve over rice.