In a bowl, toss the sliced flank steak with soy sauce, oyster sauce, cornstarch, and black pepper. Let marinate for 15 minutes.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat.
Add the marinated beef and stir-fry for 2-3 minutes until browned but not fully cooked. Remove from the pan and set aside.
Add the remaining oil to the pan, then sauté the garlic and ginger for 30 seconds until fragrant.
Add the sliced onions and stir-fry for 2 minutes until slightly softened.
Return the beef to the pan and stir to combine.
Mix in the water, sugar, and sesame oil, and stir-fry for another 2 minutes until the sauce thickens.
Add green onions, cook for another minute, then remove from heat.
Prep Time: 15 minutes | Cooking Time: 10 minutes | Total Time: 25 minutes
Kcal: 290 kcal | Servings: 4 servings
