Combine all the ingredients for the Shaking Beef Sauce and marinade with the beef. Marinate for 30 minutes, or preferably 1 to 2 hours.
Prepare the vinaigrette by mixing the rice vinegar with salt and sugar. It should have a balance of sour, salty, and sweet.
Use 3–4 tablespoons of the vinaigrette to pickle the red onions. Set aside, covered, in the fridge for about 10 minutes. Arrange the watercress and tomatoes on a serving platter and set aside.
Heat a wok, pan, or skillet over high heat. Add the cooking oil, and when it begins to smoke, add the beef cubes in an even layer. Allow to sear for about 2 minutes, then sear the opposite sides for another 1-2 minutes to brown all sides. Shake and toss the wok while cooking.
Transfer the beef to the serving platter. Drizzle the remaining 3–4 tablespoons of vinaigrette over the beef, watercress, and tomatoes. Top the beef with the pickled red onions.
In a small ramekin, make the Shaking Beef dipping sauce by squeezing lime juice over salt and black pepper. Serve the Shaking Beef immediately with the dipping sauce.