Combine soy sauce, oyster sauce, fish sauce, sugar, garlic, black pepper, and oil in a bowl. Add beef cubes, toss to coat, and marinate 20-30 minutes at room temperature.
Arrange watercress on a large platter and scatter red onion and tomato wedges over the greens.
Heat a large cast-iron skillet or wok over the highest heat until it just begins to smoke.
Add beef in a single layer without crowding (work in batches if needed). Sear about 1 minute per side, shaking and tossing the pan to brown all sides, 3-4 minutes total.
Tip the beef and its juices over the watercress. Serve immediately with lime wedges and steamed rice.