Prepare marinade by combining garlic, oyster sauce, sugar, fish sauce, thick soy sauce and sesame oil with the beef for at least half an hour, preferably 1-2 hrs.
Prepare vinaigrette by mixing rice vinegar with salt and sugar. Adjust to taste. It should be a balance of sour, salty and sweet.
Thinly slice the red onion and use about 2 tbs of the vinaigrette to pickle and set aside covered in fridge for about 10 minutes. Prepare bed of watercress and tomatoes in a serving platter and set aside.
Heat a large wok or pan over high heat. Add about 2 tbs cooking oil and when it begins to smoke, add an even layer of beef and allow to sear for about 2 minutes, before “shaking” to sear the opposite sides for about another 1-2 minute more to brown all the sides. Do this in batches to cook all the beef if necessary.
Transfer beef to bed of watercress and tomatoes. Drizzle another 3-4 tbs of vinaigrette over the beef and greens and top with pickled red onions. Lastly, squeeze lime juice over salt and pepper in a small ramekin.