In a large bowl combine the garlic, ginger, soy sauce, mirin and sugar.  Add the beef strips and marinate for 30 minutes.  Drain the beef strips, reserving the marinade.

In a small bowl combine the cornflour and 1/3 cup cold water and mix into a smooth paste, set aside.

Cook the beef in batches in a frypan over high heat, will only need 1-2 minutes on each side, reduce heat and continue until cooked to your liking.

While the beef is cooking, place the marinade into a small saucepan and bring to the boil.  Add the cornflour and stir until it returns to the boil.

Remove sauce from heat and pour over beef to serve.