Prepare the marinade by whisking together soy sauce, oyster sauce (if using), grated ginger, minced garlic, and sesame oil in a bowl.

Marinate the beef strips by tossing them in the marinade, then letting them sit for 10-15 minutes.

Cook the onions by heating 1 tablespoon of vegetable oil in a skillet over medium heat. Add the sliced onion and sauté until soft and caramelized, which takes about 8-10 minutes. Season with salt and pepper, then set aside.

Sear the beef by increasing the heat to high and adding 2 tablespoons of vegetable oil to the skillet. Add the marinated beef and stir-fry for 2-3 minutes until browned.

Make the sauce by combining soy sauce, oyster sauce (if using), hoisin sauce, rice vinegar, and cornstarch in a bowl until smooth.

Thicken the sauce by pouring the mixture over the beef in the skillet, stirring, and letting it simmer for 1-2 minutes until thickened.

Combine the sautéed onions back into the pan, tossing everything together and cooking for another 30 seconds.

Serve by transferring the mixture to a platter, enjoying it hot over rice or noodles.