Slice beefsteak across the grain into thin strips. Place them in a bowl. Blend cornflour, salt, freshly ground black pepper, soy sauce, oil, sherry and vinegar, and pour over beef. Chop the onions very finely and sprinkle over top. Cover tightly and let stand for at least 1 hour.
When ready to cook: place bowl of beef on a stand over a saucepan of boiling water; cover saucepan and steam