Sprinkle 3 tbsp cornstarch on the marinated beef. Mix until evenly coated.
Heat a pan up to medium-high. Add ⅓ cup oil and wait for the oil to heat up (approximately 2-3 minutes).
Add the coated beef to the pan. Toss (keeping beef separated into strips) and cook for 2 minutes per-side or until golden/crisp. Remove the cooked beef, place on a plate lined with a paper towel (to drain the grease) and set aside.
Remove most of the oil from the pan, leaving at least 1 tsp for reuse.
Add garlic to the pan and saute/cook for 30 seconds or until fragrant.
Return the cooked beef to the pan, pour in the sauce mixture, and toss/mix for 2-3 minutes or until the sauce has almost completly evaporated.
Add cut green onions, then toss/mix. Remove the pan from the heat.