In a mixing bowl, combine the oyster sauce, hot water, sesame oil, and sugar. Add the beef and marinate for 10 minutes.
Heat the wok over high heat until smoking hot. Add the vegetable oil, then add the beef and sear on each side for 1 minute, the beef should be charred and the wok flaming.
Add the garlic, onions, and butter to the wok and continue to stir-fry for a further 2 minutes. The beef should be medium-rare.
Serve with jasmine rice or baguette and dip into soy sauce and chile sauce.
If you don't have a large wok, then cook the beef in two batches. This will ensure that the heat will stay at a constant high.
Recipe courtesy of Luke Nguyen