Slice steak very thinly. Place in a bowl with soy sauce, cornstarch and a few drops of water. Mix well and set aside.
Slice onion very thinly. Cut broccoli into small pieces and steam until partly cooked. Drain and set aside.
Heat oil in skillet until very hot. Add garlic and ginger. Cook 1 minute, then remove. Add meat and stir fry until meat sweats, then add onion and broccoli. Stir fry until vegetables are crispy tender.
Stir in oyster sauce, a pinch of brown sugar, salt and 1/2 cup of cold water with 1 Tblsp cornstarch dissolved in it. Stir and cook until sauce is thickened.