Prepare marinade by combining garlic, oyster sauce, sugar, fish sauce, thick soy sauce and sesame oil with the beef for 10 mins
Prepare vinaigrette by mixing rice vinegar with salt and sugar. It should be a balance of sour, salty and sweet.
Thinly slice the red onion and use about 3-4 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.