In a mixing bowl, combine the beef cubes with fish sauce, oyster sauce, sweet soy sauce, sugar, sesame oil, and salt. Mix well to ensure each piece of beef is coated. Let it marinate for at least 30 minutes, or up to 2 hours in the refrigerator for more intense flavor.
While the beef is marinating, slice the red onion and tomatoes. Trim the watercress stems and set aside. This will ensure everything is ready to go when you start cooking.
Heat the large skillet or wok over medium-high heat. Add the cooking oil and allow it to get hot. You want the oil shimmering but not smoking.
Add the marinated beef to the skillet in a single layer. Let it sear without moving it for about 2 minutes to develop a nice crust. Then, use the spatula to shake and toss the beef, cooking for an additional 2-3 minutes until browned and cooked to your desired doneness.
Add the minced garlic and sliced red onion to the skillet. Stir-fry everything together for about 2 minutes until the onions are translucent and fragrant.
Sprinkle the black pepper over the mixture, add the rice vinegar, and give everything a final toss. Remove the skillet from heat.
Plate the beef mixture and garnish with fresh watercress and sliced tomatoes. Serve with lime wedges on the side for an extra burst of flavor.
