
In a medium bowl, whisk together the soy sauce, water, brown sugar, ginger, and garlic. Set aside.
In a large bowl, toss the steak with the cornstarch until well coated.
In a large skillet or wok, heat the oil over medium-high heat. Shake the beef strips to remove excess cornstarch, and then add the beef to the skillet. Cook, stirring occasionally, until the beef is browned and mostly cooked through, 3 to 4 minutes. Transfer the beef to a large bowl.
With the skillet still over medium-high heat, add the sauce to the skillet and cook, stirring occasionally, scraping any browned bits from the bottom of the skillet with a spoon, until the sauce is thickened, about 3 minutes. Stir in the beef, red chilies (if using), and green onions.
Cook, stirring often, until the beef is well coated and the green onions are softened, about 1 minute. Serve immediately over rice.