Gently combine all salad ingredients and arrange on a platter.
To prepare the beef: Combine the oyster sauce, soy sauce and beef in a medium bowl and toss to coat the meat evenly.
Heat the oil in a skillet over high heat. Add the garlic and stir-fry quickly until fragrant, about 10 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 dressing. Season with salt and pepper.
Add the pineapple and Asian basil to the beef mixture.
Arrange the beef mixture and all its juices on the watercress and serve immediately.