Season steak with salt and pepper. Place in preheated grill pan that has been sprayed with cooking spray. Sear on each side for 2 to 3 minutes for medium-rare, or cook to desired degree of doneness. Remove from pan and let rest before slicing against the grain on the bias.
Place lettuce on 4 individual plates. Top lettuce with watercress and bean sprouts. Top with steak slices, tomatoes, cucumber, and scallions. Set aside.
In a small bowl, whisk together peanut sauce and lime juice. Pour mixture over salad. Top with steak and sprinkle peanuts over top just before serving.