Trim the broccoli and cut the heads off very close to the tips. Slice the stalks across on the diagonal into quarter inch pieces. Split the head into pieces no bigger than a walnut. Peel and finely chop the garlic and ginger. Heat the oil in a large frying pan or wok and fry the garlic and ginger for one minute. Add the broccoli stalks and fry for a minute and a half. Add two Tablespoons of water - be careful, it will spit and give off lots of steam - then add the broccoli florets. They will turn a wonderful bright green. Stir fry for another two minutes over a medium to high heat until the vegetables are cooked and the liquid has vanished. Remove the pan from the heat and pour the oyster or soy sauce over the broccoli, turning it so it is well coated in the rich, dark sauce. Check for seasoning and serve really hot.