01 - Combine soy sauce, oyster sauce, Shaoxing wine, black pepper, cornstarch, sugar, water, and sesame oil in a small bowl. Stir until smooth and set aside.
02 - Heat 1 tablespoon vegetable oil in a wok or large skillet over high heat. Add the steak strips and stir-fry for 2 to 3 minutes until browned but still tender. Transfer beef to a plate.
03 - Add remaining 1 tablespoon vegetable oil to the pan. Stir-fry sliced onion and minced garlic for 1 minute until fragrant.
04 - Add red and green bell peppers to the pan and stir-fry for 2 to 3 minutes until slightly tender yet crisp.
05 - Return the cooked beef to the pan, pour in the prepared sauce, and toss to coat evenly. Cook for another 1 to 2 minutes until the sauce thickens slightly and ingredients are heated through.
06 - Remove from heat, sprinkle with sliced spring onions, and serve immediately, ideally alongside steamed rice.