Sprinkle the sirloin with soy sauce and rub it into the surface. Broil or grill until done to your liking, about 5 minutes on each side for rare.
Let the steak relax for 10 minutes, then slice thinly against the grain.
Toss with the onion and cucumber and red bell pepper.
In a small bowl combine the lime juice, fish sauce, garlic and hot pepper sauce. Taste dressing and add sugar to taste. Pour over the salad an toss well to combine. Arrange the salad on lettuce and serve warm.