Slice the Onion thinly and cut the Green Onion into 2-inch pieces; mince the Garlic.
Slice the Beef thinly against the grain and place in a bowl for marinating.
To the beef, add 1 tsp Salt, 1 tsp Sugar, 1/4 tsp Baking Soda (to tenderize), 1 Tbsp Corn Starch, 1 Tbsp Soy Sauce, 1 tsp Dark Soy Sauce, 1 tsp Sesame Oil and 1 Tbsp Cooking Wine; mix well and let rest for 15–20 minutes.
Heat a wok or large skillet over high heat and add 1–2 Tbsp Oil (for cooking). When the oil is hot, quickly stir-fry the marinated beef in batches until just browned (about 1–2 minutes per batch). Remove the beef and set aside.
Add another small drizzle of Oil to the pan if needed, then stir-fry the sliced Onion until it begins to soften, about 2–3 minutes.
Add the minced Garlic and the Green Onion and stir-fry briefly until fragrant, about 30 seconds.
Return the beef to the wok, then add 1 Tbsp Oyster Sauce, 1 tsp Dark Soy Sauce, 1 tsp Sugar (adjust to taste), a splash (2–3 Tbsp) of Water to loosen the sauce, and a grind of Black Pepper; toss everything to combine and cook for another minute so the flavors meld and the sauce thickens slightly.
Taste and adjust seasoning with more Soy Sauce or Salt if needed. Serve immediately over steamed rice.
