Prepare the dressing by combining the fish sauce, lime juice, brown sugar and chopped chiles in a small bowl; set aside.
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.
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.
Place mixture on lettuce leaves, and serve immediately.
