To make the marinade, combine the garlic, oyster sauce, sugar, fish sauce, soy sauce and sesame oil. Marinate the beef for at least half an hour.
Prepare vinaigrette by mixing rice vinegar with salt and sugar.
Thinly slice the red onion and put in a shallow dish. Pour 4 tbsp of the vinaigrette over the onions and place in fridge for about 10 minutes. Set the remaining vinaigrette aside.
Take the trimmed watercress and spread it on the serving plate. Top it off with the sliced tomatoes and set aside.
Prepare the dipping sauce by squeezing the lime juice over the kosher salt and pepper in a small dish.
In a large wok or skillet heat the oil over high heat. When it begins to smoke, add an even layer of beef and allow to sear untouched for about 2 minutes. Turn the pieces over to sear the other side for another 2 minutes. If you want the beef well done, stir and cook for an additional 2 minutes.
Transfer beef to bed of watercress and tomatoes. Drizzle 4 tbsp of remaining vinaigrette over the beef and greens and top with the red onions.