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. 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.
For the beef: Heat a wok over high heat and fill halfway up with peanut oil. Heat the oil to 350 degrees F, or until a cube of bread turns golden brown in 15 seconds and floats to the surface.
Place the beef strips in a large bowl and add 2 tablespoons cornstarch. Toss until the beef has absorbed the cornstarch. Add the remaining 1 tablespoon cornstarch right before frying and toss to coat.
Fry the beef in 2 to 3 batches until golden, 4 to 5 minutes. Drain on paper towels and season with sea salt.
For the sauce: Set another wok or saute pan over high heat and add the soy sauce, chili sauce and orange juice.
Bring to a simmer and cook until it becomes thickened and coats the back of a spoon, 1 to 2 minutes.
Toss the beef in the sauce to coat thoroughly. Serve over a bed of the shredded lettuce and sliced cucumber that has been sprinkled with rice vinegar. Then garnish with the scallions and orange zest.