Season the fish with garlic powder, salt and pepper; cook in microwave until slightly cooked, about 2 minutes. Mix together the spring onion, capsicum, coriander, sour cream, mayonnaise and 1 cup Cheddar cheese in a bowl. Gently fold the fish into the mixture.
Prepare a 20x30cm baking dish with cooking spray. Pour 1/4 of the enchilada sauce into the bottom of the baking dish.
Pour 1/2 enchilada sauce into a bowl. Dip the tortillas, one at a time, into the sauce to lightly coat. Spoon the fish in equal portions into the centre of each tortilla. Make the enchiladas by tightly rolling the tortillas and placing seam-side down in the baking dish. Pour the remaining enchilada sauce over top of the enchiladas. Sprinkle with 1 cup Cheddar cheese. Cover the dish with aluminium foil.
Bake in preheated oven for 45 minutes. Top with avocado slices to serve.