Thinly slice beef against the grain into 0.75 cm thick pieces and about 2 inches in length. Do not slice any thinner.
Marinate beef with marinade ingredients and set aside.
In a bowl, combine sauce ingredients.
Generously dredge several beef slices into cornstarch until well coated and transfer to a plate. Repeat this process until all your beef is coated.
In a large pan set over medium heat, heat oil. Check if oil is hot by inserting a wooden chopstick and look for bubbles.
Once oil is hot, fry beef in batches, 1 minute on each side until slightly crispy. Important: Immediately spread beef apart to prevent sticking. Fry in 2-3 batches to prevent overcrowding.
Remove cooked beef from oil and set aside. Discard excess oil leaving 1 tsp in the pan.
Over low heat, sauté ginger, garlic, red chili for 10 seconds. Pour in sauce to reduce and thicken.
Once thick, add beef and mix to coat evenly. Garnish with green onions. Remove off heat. Optionally garnish with sesame seeds. Enjoy!