01 - Combine beef strips with cornstarch and 1 tablespoon soy sauce in a bowl and let marinate for 10 minutes.
02 - Whisk together soy sauce, oyster sauce, hoisin sauce, rice vinegar, chili garlic sauce, sugar, and water in a small bowl. Set aside.
03 - Warm 1 tablespoon vegetable oil in a large wok or skillet over high heat.
04 - Add marinated beef to the wok and stir-fry for 2 to 3 minutes until browned but still tender. Remove beef and set aside.
05 - Add bell peppers, red onion, garlic, and ginger to the wok; stir-fry for 2 to 3 minutes until vegetables are crisp-tender.
06 - Return beef to the wok, pour in the prepared sauce, and toss everything together. Stir-fry for 1 to 2 minutes until the sauce thickens and coats all ingredients evenly.
07 - Plate stir fry hot, garnished with sliced spring onions and sesame seeds. Serve alongside steamed rice or noodles as desired.