Combine the lime juice, fish sauce, sugar and chiles in a large bowl. Set aside.
Place the oyster sauce, soy sauce and beef in a medium bowl and toss to coat the meat evenly.
Heat the oil in a large skillet over high heat. Add the garlic and stir until fragrant, about 5 seconds. Add the beef and quickly stir-fry it until just charred on the edges, about 2 to 3 minutes. Add to the bowl with the lime sauce.
Add the pineapple slices, if using, Asian basil and red onion to the beef mixture. Spread the watercress and tomatoes in an attractive manner on a serving platter. Arrange the beef mixture and all its juices on the watercress and serve immediately (if desired, with a warm baguette or steamed rice to soak up the juices).
Another way of presenting bo luc lac is to omit the lime dressing completely and serve the beef with a dipping salt mixture made with 1 1/2 tablespoons sea salt, 1 teaspoon freshly ground black pepper, and 1 tablespoon fresh lime juice.
Recipe courtesy of Mai Pham's Pleasures of the Vietnamese Table (HarperCollins, 2001)