Season the steak with salt and set aside. Heat a gas or charcoal grill over high heat until very hot. Brush the steak with oil, and grill to desired doneness, 4-5 minutes per side for medium-rare. Transfer to a cutting board and let rest for 5 minutes.
In a small bowl, whisk together the lime juice, fish sauce, and sugar.
Tear the lettuce into bite-size pieces, and place in a large mixing bowl. Roughly tear the mint and cilantro and add to the bowl. Add the cucumber, tomatoes, and red onion.
Trim away any excess fat from the steak, and thinly slice it across the grain. Pour the accumulated juices into the dressing, and mix. Drizzle 4 tablespoons of dressing over the lettuce mixture, and toss. Add more dressing if needed.
Divide the salad between 4 plates, and top each with sliced steak. Drizzle a little more dressing over the steak before serving.