01 - Combine beef strips with soy sauce, cornstarch, and sesame oil; let marinate for 10 minutes.
02 - Whisk together soy sauce, oyster sauce, chili garlic sauce, brown sugar, water, and rice vinegar in a small bowl; set aside.
03 - Heat 1 tablespoon vegetable oil in a large wok or skillet over high heat. Stir fry marinated beef for 2 to 3 minutes until browned; remove and set aside.
04 - Add remaining oil to wok. Stir fry garlic, ginger, and onion for 1 minute until fragrant.
05 - Add red and green bell peppers; stir fry for 2 to 3 minutes until slightly tender.
06 - Return beef to the wok, pour in sauce, and toss for 1 to 2 minutes until fully heated and coated.
07 - Stir in sliced spring onions; remove from heat.
08 - Serve immediately garnished with extra spring onions if desired.