Coat the thinly sliced beef strips with the cornstarch generously, dust off any excess and set aside.
In a large skillet, heat half of the vegetable oil, toss in the minced garlic and ginger and fry for 1-2 minutes till fragrant.
Pour in the soy sauce, water and brown sugar, stir and let it come to a soft boil. Let simmer for 5 minutes, the sauce will lightly thicken.
Transfer the sauce into another bowl, leave to cool down while you continue cooking.
In the same heated pan, pour in the leftover oil. Toss in the beef strips and start to saute on high heat until brown, this should take about 10 minutes while continuously moving the beef around to prevent burning.
Add in the soy sauce mix back into the browned beef, cover the pan with the lid, reduce the heat and let it cook slowly for 12 minutes, the sauce will begin to thicken.
Turn off the heat and garnish with the green onions.
Serve hot with some white rice.