1. Preheat oven to 400 degrees F (200 degrees C).
2. Prepare the dressing by combining the fish sauce, lime juice, brown sugar and chopped chiles in a small bowl; set aside.
3. Place the salmon filet on a baking tray, and rub olive oil over. Bake for 20 minutes in the preheated oven, or until easily flaked. Let cool for at least 15 minutes.
4. Place salmon filet into a big bowl; use fork to break salmon meat into big chunks. Add onion, tomato and basil; pour dressing over, and toss lightly until well mixed.
5. Place mixture on lettuce leaves, and serve immediately.