Mix the thinly sliced beef with cooking wine, 2 Tbsp of soy sauce, and corn starch. Let it marinade for about 30 minutes
When ready to cook, preheat the wok or skillet until really hot, add in about 2 Tbsp of oil and add in beef slices and stir-fry until they turn color but not overcooked yet. Remove from the wok and set aside. Add in about 2 Tbsp of oil. Stir-fry the onion until really fragrant, about 2 minutes. Return the beef back into the wok and add in the sauce. Stir-fry for another 2 minutes or so. Transfer to serving plate and serve immediately