Preheat oven to 425 degrees.
In a small bowl, combine seasonings (chili powder, salt and pepper); rub mixture into both sides of fish then place it in a baking dish. Bake for 15 to 18 minutes or until fish flakes easily when tested with a fork. Reset oven temperature to 325 degrees.
Allow fish to cool for 5 minutes then cut it into cubes and place in a medium bowl; add onion, cilantro, lime juice and chilies; toss well.
Fill tortillas with fish mixture and top with cheese; place on a baking sheet and bake for 5 minutes until cheese has melted. Serve with diced avocado.