In a bowl, mix soy sauce, oyster sauce, garlic, ginger, and sesame oil. Add the beef strips and marinate for 10–15 minutes.
Heat 1 tablespoon of oil in a pan over medium heat. Add sliced onions and sauté until soft and lightly caramelized. Season with salt and pepper, then remove and set aside.
In the same pan, heat 2 tablespoons of oil over high heat. Add the marinated beef and stir-fry for 2–3 minutes until browned.
In a separate bowl, whisk together soy sauce, hoisin sauce, rice vinegar, and cornstarch.
Pour the sauce over the beef and let it simmer until thickened.
Return the onions to the pan and toss everything together.
Serve immediately while hot and sizzling for the best Sizzling Chinese Pepper Steak with Onions experience.