01 - Toss the sliced flank steak with 2 tablespoons cornstarch in a medium bowl until evenly coated. Set aside while preparing the sauce.
02 - Whisk together soy sauce, 1/4 cup water, brown sugar, grated ginger, minced garlic, oyster sauce, and sesame oil in a small bowl. In a separate bowl, mix 1 tablespoon cornstarch with 2 tablespoons water to create a slurry.
03 - Heat 1 tablespoon vegetable oil in a large skillet or wok over medium-high heat. Add the beef in a single layer and sear for 1-2 minutes per side until browned but not fully cooked through. Remove beef from the pan and set aside.
04 - Add the remaining tablespoon of oil to the skillet. Add broccoli florets and sliced red bell pepper. Stir fry for 3-4 minutes until vegetables are bright green and just tender.
05 - Return the beef to the skillet. Pour in the ginger sauce and bring to a simmer. Stir in the cornstarch slurry and cook, stirring constantly, until the sauce thickens and coats the beef and vegetables, about 2-3 minutes.
06 - Sprinkle with sliced green onions and serve immediately over steamed jasmine or brown rice.