Cut the steak, against the grain, into thin shred. Mix together the sherry, salt, pepper, soy sauce, sugar and 1 ½ tsps of the oil in a shallow dish. Add the steak shreds and leave to marinate for 15 minutes.
Heat the 2 ½ tbsps of the remaining oil in a wok or pan over high heat. Add the ginger and onion and stir fry for 1 ½ minutes. Push them to one side and add the remaining oil to the other side of the pan. When it is very hot, add the steak and garlic.
Stir fry the steak shred and garlic in the oil for a few seconds until brown. Mix in the onion and ginger and stir fry for a further 1 minute. Stir in the cornstarch mixture and cook, stirring for about 30 seconds or until thickened.