Make the marinade: Whisk together 1 tbsp soy sauce, 1 tbsp oyster sauce if using, grated ginger, minced garlic, and sesame oil in a bowl.

Marinate the beef: Toss the beef strips in the marinade and let sit for 10 to 15 minutes at room temperature.

Cook the onions: Heat 1 tbsp vegetable oil in a skillet over medium heat. Add the sliced onion and cook until soft and slightly caramelized, about 8 to 10 minutes. Transfer the onions to a bowl.

Sear the beef: Wipe the skillet if needed. Heat 2 tbsp vegetable oil over high heat. Add the marinated beef in a single layer and stir-fry for 2 to 3 minutes until browned. Work quickly to avoid overcooking.

Make the sauce: In a small bowl, whisk together 2 tbsp soy sauce, 2 tbsp oyster sauce if using, 2 tbsp hoisin sauce, 2 tbsp rice vinegar, and 1 tbsp cornstarch until smooth.

Combine: Pour the sauce into the pan with the beef. Reduce heat to medium and simmer for 1 to 2 minutes until it thickens and becomes glossy.

Finish: Return the sautéed onions to the pan. Toss to coat and heat through. Adjust seasoning with salt and pepper to taste.

Serve: Transfer to a platter and serve hot over steamed rice or noodles.