Add 1 ½-2 pounds flank steak sliced into ¼-inch thick strips, and ⅓ cup cornstarch to a large ziplock bag. Close the bag and shake vigorously until all the beef is evenly coated in cornstarch. Set aside for about 15 minutes.
Heat a large 14-inch skillet over medium-high heat and add 1 tablespoon of the 2 tablespoons vegetable oil. Add half of the beef to the skillet and cook it undisturbed for 3-4 minutes on both sides until browned. Remove the beef to a plate and repeat with the second half of the beef.
Add the last tablespoon of vegetable oil and ½ teaspoon sesame oil to the skillet and heat over medium heat. Add 2 teaspoons minced garlic and 1 teaspoon fresh minced ginger and cook for 1 minute, until fragrant.
Whisk ½ cup low-sodium soy sauce, ⅓ cup lightly packed light brown sugar, ⅓ cup water, and 1 tablespoon rice vinegar together in a medium bowl to make the sauce. Add the sauce to the pan and cook until the brown sugar is completely dissolved, about 2-3 minutes.
Combine 1 tablespoon cornstarch and 2 tablespoons water in a small bowl to make a slurry. Stir the slurry into the sauce, then simmer over medium-low heat for 1-2 minutes until thickened.
Add the beef back to the skillet and stir until coated. Cook for 1-2 minutes until completely heated through, then serve immediately over white rice. Garnish with green onions and sesame seeds, if desired.
Video
Notes
Store & Reheat: Store leftovers in the fridge for up to 5 days or freeze for up to 3 months. Reheat in the microwave or a skillet until hot and steaming.