In a medium bowl, marinate the sliced beef with the ingredients listed under marinade. Cover and set aside. You can also refrigerate this for a longer marination until ready to use.
Meanwhile, combine the ingredients for the sauce. Keep the rest of the ingredients for stir-frying near the stovetop. This is also a good time to prepare the rice if you haven't already.
Heat 3 tbsp of the oil in a wok or skillet over high heat. Use tongs to lift half the meat pieces and place them into the hot oil. Brown them over high heat for about 1-2 minutes on each side. (Note 1) Remove and set aside on a plate. Repeat with the remaining beef.
In the same pan, heat the remaining 1 tbsp of oil and add the onions. Sauté until they are slightly brown, about 3 minutes. Add the garlic and ginger and continue to sauté for a minute.
Pour in the sauce and mix well. Let this simmer for about 10-15 seconds, stirring constantly, until slightly thickened. Add the cooked beef and stir-fry for a couple of minutes until the beef is cooked through and the sauce has reduced down to your desired consistency. Taste and adjust seasoning, adding salt if needed or red chili flakes to make it spicier.
Mix in the green onions. Sprinkle with sesame seeds, if using. Serve immediately with white rice.
Notes
See tips within blog post text. Note 1: Ideally, stir-fry in two batches to prevent overcrowding and lowering the heat in the pan. High heat helps brown the outside and seal in the juices. Also be sure not to overcook the meat, as you want it to remain as tender as possible.