1. Combine the lime juice, fish sauce, sugar, and half the chopped fresh mint in a small bowl. Spoon 2 tablespoons of the dressing onto the flank steak; turn the steak over to coat both sides.
2. Broil steak 7 minutes per side, depending on thickness, until it achieves desired doneness. Let the steak stand 5 minutes; cut on a diagonal into very thin slices.
3. While steak cooks, toss remaining dressing with the carrot, onion, cucumber, red bell pepper, jalapeño, and remaining mint. Add the sliced steak and peanuts. Place 1 cup of the watercress on each of 4 plates. Top each plate with 1/4 steak mixture.