In a bowl, whisk together soy sauce, oyster sauce (if using), ginger, garlic, and sesame oil. Toss the beef strips in this mixture and let them sit for about 10–15 minutes.
Heat 1 tablespoon of vegetable oil in a skillet over medium heat. Add the sliced onions and sauté until soft and slightly caramelized. Season with salt and pepper before removing from the pan.
In the same pan, increase the heat to high and add another 2 tablespoons of vegetable oil. Once hot, add the marinated beef strips and stir-fry for about 2–3 minutes until browned.
Prepare the sauce by whisking together soy sauce, hoisin sauce, rice vinegar, and cornstarch in a small bowl. Pour this mixture over the browned beef and let it simmer briefly until thickened.
Add the sautéed onions back into the pan with the beef mixture, stirring gently until well combined. Serve hot over steamed rice or noodles.