Prep: 20 minutes; Cook: 14 minutes.
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.
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.
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.