Place the thinly sliced flank steak in a medium bowl. Sprinkle the cornstarch over the beef and toss to coat evenly.

Heat the oil in a large skillet or wok over medium-high heat. Once the oil is hot, add the beef in a single layer (you may need to do this in batches). Sear the beef for about 2-3 minutes per side, until it's browned and slightly crispy. Remove the beef from the skillet and set aside.

In the same skillet, add the minced garlic and ginger. Sauté for about 30 seconds, or until fragrant.

Add the soy sauce, water, brown sugar, and red pepper flakes (if using) to the skillet. Stir well to combine. Bring the mixture to a simmer, stirring constantly, until the sugar is dissolved and the sauce has thickened slightly, about 3-5 minutes.

Return the beef to the skillet. Toss to coat the beef evenly with the sauce. Cook for another 1-2 minutes, allowing the sauce to further thicken and cling to the beef.

Remove the skillet from the heat. Garnish with sliced green onions. Serve immediately over steamed rice.