In a bowl, whisk together soy sauce, oyster sauce, ginger, garlic, and sesame oil.
Toss the beef strips in the marinade and let sit for 10-15 minutes.
Heat 1 tbsp vegetable oil in a skillet over medium heat.
Add sliced onions and sauté until soft and slightly caramelized.
Season with salt and pepper, then remove from the pan and set aside.
In the same pan, heat 2 tbsp vegetable oil over high heat.
Add the marinated beef strips and stir-fry for 2-3 minutes until browned.
In a small bowl, whisk together soy sauce, hoisin sauce, rice vinegar, and cornstarch.
Pour the sauce over the beef and let simmer until it thickens.
Add the sautéed onions back into the pan, stirring to combine.
Serve hot over steamed rice or noodles.