Slice the beef thinly and marinate it with soy sauce and cornstarch for 15–20 minutes.
Heat oil in a wok or skillet and stir-fry the beef until lightly browned. 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, then simmer until the sauce thickens slightly.
Return the beef to the pan and toss until evenly coated with the teriyaki sauce.
Sprinkle with sesame seeds and sliced green onions, then cook for another minute.
Serve hot with steamed rice or noodles.