1. Bring 3 quarts of water to a boil in a large saucepan. Add beef, reduce heat to medium-high, and simmer 25 minutes or until medium-rare. Remove beef from saucepan with tongs, cover beef with aluminum foil, and let stand 10 minutes.
2. Combine canola oil, lime juice, fish sauce, soy sauce, sugar, and red pepper flakes in a small bowl, stirring with a whisk. Set aside.
3. Combine watercress, mint, cilantro, radishes, onion, and lemon rind in a large bowl; refrigerate, covered, until ready to serve.
4. Cut beef crosswise into 1/2-inch thick slices, then into long strips. Add beef and dressing to watercress mixture, and toss gently to coat.