Slice the beef thinly and coat it with cornstarch. Let it rest for 15–20 minutes.
Heat oil in a wok or skillet and stir-fry the beef until browned and slightly crispy. Remove and set aside.
In the same pan, sauté minced garlic and grated ginger until fragrant.
Add soy sauce, brown sugar, and a splash of water. Simmer until the sauce begins to thicken.
Return the cooked beef to the pan and toss until evenly coated in the glossy sauce.
Stir in chopped green onions and cook for another minute.
Serve hot with steamed rice or noodles.