In a mixing bowl, add soy sauce, oyster sauce (if using), grated ginger, minced garlic, and sesame oil. Whisk these ingredients together to create a marinade.

Place the beef strips in the bowl, ensuring they are well-coated in the marinade. Let the beef sit in the marinade for about 10-15 minutes.

While the beef marinates, heat 1 tablespoon of vegetable oil in a skillet over medium heat.

Add the sliced onions to the skillet. Sauté them until they become soft and slightly caramelized, which should take about 5-7 minutes.

Season with salt and pepper to taste, then remove the onions from the pan and set them aside.

Using the same skillet, increase the heat to high and add 2 tablespoons of vegetable oil.

Once the oil is hot, add the marinated beef strips. Stir-fry the beef for about 2-3 minutes, or until it is browned and cooked through.

In a small bowl, whisk together the soy sauce, hoisin sauce, rice vinegar, and cornstarch.

Pour this sauce over the beef in the skillet and stir well. Allow it to simmer for a few minutes until the sauce thickens.

Carefully add the sautéed onions back into the skillet, stirring to combine everything evenly.

Serve the sizzling beef and onions hot over a bed of steamed rice or noodles.