Thai Beef Salad

In 8-inch or 9-inch square glass baking dish, stir 1 tablespoon fish sauce and 1 teaspoon sugar. Add steak, turning to coat; marinate 15 minutes at room temperature or 1 hour in refrigerator, turning occasionally. Prepare charcoal fire or preheat gas grill for covered direct grilling over medium heat. Meanwhile, from limes, with vegetable peeler, remove peel in 2-inch by 3/4-inch strips. With sharp knife, cut enough peel crosswise into matchstick-thin strips to equal 1 tablespoon. Squeeze limes to equal 3 tablespoons juice. In small bowl, whisk lime juice, oil, crushed red pepper, black pepper, the remaining 1 tablespoon fish sauce, and 1 1/2 teaspoons sugar until blended. In large bowl, toss watercress, mint, cilantro, radishes, onion, and lime peel; cover and refrigerate until ready to serve. Place steak on hot grill rack. Cover grill and cook steak 10 to 15 minutes for medium-rare or until desired doneness, turning over once. Transfer steak to cutting board; let stand 10 minutes to set juices for easier slicing. Cut steak diagonally into thin strips. Add steak and dressing to watercress mixture and toss until well coated.