Mix all the MARINADE ingredients in a bowl and combine with the sliced beef. Marinate for at least 20 minutes or overnight.
Combine all the SAUCE ingredients in a small bowl and set aside.
Heat a tablespoon of oil in a wok or skillet over high heat. Add the beef in a single layer and brown on both sides. No need to cook it all the way through, just a quick sear. Remove from the wok and set aside.
Add a bit more oil to coat the wok and turn the heat down to medium. Add garlic and stir fry until it becomes fragrant, about 5-10 seconds. Be careful since garlic can burn quickly.
Add the scallions, turn the heat to high, and stir fry until it begins to wilt.
Return the beef to the wok, pour in the sauce and toss well to combine.
Stir fry over high heat for about 45-60 seconds, just long enough for the sauce to thicken and cling to the beef. Serve with rice.