In a bowl, whisk together soy sauce, oyster sauce, ginger, garlic, and sesame oil until combined well. Toss the beef strips in this marinade, ensuring they are fully coated, and let them sit for about 10-15 minutes to absorb all those rich flavors.
Heat 1 tablespoon of vegetable oil in a skillet over medium heat until shimmering but not smoking. Add sliced onions to the pan and sauté them until they become soft and slightly caramelized, which should take around 5-7 minutes, then season lightly with salt and pepper before removing them from the pan.
Using the same skillet, increase the heat to high and add 2 tablespoons of vegetable oil until hot. Carefully add in your marinated beef strips in batches if necessary so they can sear properly without overcrowding, stir-frying them for about 2-3 minutes until browned on all sides.
In a small bowl while beef is cooking, whisk together additional soy sauce, hoisin sauce, rice vinegar, and cornstarch until smooth without lumps forming. Pour this mixture over your cooked beef just as it starts browning well; let it simmer briefly while stirring until it thickens up nicely.
Return your sautéed onions back into the pan with beef and sauce; stir everything together gently but thoroughly so all ingredients are well mixed and heated through before serving hot over steamed rice or noodles as desired.