Trim and slice steak across the grain into strips about 2 inches long and 1/8 inch wide. Cut green onions diagonally into 1 inch pieces.
In wok or large skillet, heat oil over high heat, just until starting to smoke. Stir fry steak, garlic, pepper & hot pepper flakes for 2 minutes until steak is still slightly pink.
Add green onions, soy sauce, sherry, sugar and sesame seeds; bring to boil. Push meat mixture to the side.
Blend cornstarch into water; add to liquid in center of wok and cook, stirring until thickened. Returtn meat mixture to sauce; toss to glaze.
