Slice the sirloin steak into thin, narrow strips.
In a medium bowl, combine the ingredients for the marinade. Add the sliced beef and stir to combine. Set aside.
Meanwhile, prepare the bitter melon by slicing in half, lengthwise. Remove the seeds with a spoon and then slice each half crosswise as thinly as possible.
Using a non-stick skillet, heat the oil over medium high heat. Add the garlic and saute for 30 seconds. Add the marinated beef and stir-fry until the beef is no longer pink, around 3 minutes. Add 1 tablespoon oyster sauce and stir to coat.
Stir in the bitter melon. Add the water and the remaining 2 tablespoons oyster sauce. Stir and then cook, covered until the bitter melon is crisp tender, around 3 minutes. Turn off the heat and let the skillet sit, covered for 5 minutes.
Transfer to a serving plate and serve with steamed rice.