Combine the fish sauce, soy sauce, sugar and garlic in a medium bowl. Add the beef and stir to coat. Season with pepper. Cover and set aside for 1 hour to marinate.
Heat a wok over high heat. Add half the oil and heat until smoking. Cook half the beef, shaking the wok, for 1-2 minutes until the beef is charred but still rare in the centre, or until cooked to your liking. Transfer to a serving platter. Repeat with the remaining oil and beef, reheating the wok between batches. Top with watercress, tomato and onion.