Cut the broccoli heads into small florets and blanch in boiling water for 3-4 minutes, plunge into cold water and drain well. Blend the cornflour with the stock.
Heat the oil in a wok or large frying pan and add the broccoli and garlic. Stir-fry for 1 minute, then add the oyster sauce and cook for 1 minute. Add the stock and cornflour mixture and as soon as it thickens, stir in the sesame oil. Sprinkle with the sesame seeds and serve immediately.