Season the tilapia with garlic powder, and pepper; cook in microwave until slightly cooked, about 2 minutes. Mix together the green onion, bell pepper, cilantro, sour cream, mayonnaise and 1 cup Cheddar cheese in a bowl. Gently fold the tilapia into the mixture.
Prepare a 9x13 baking dish by pouring 1/2 of a can of enchilada sauce into the bottom of the baking dish.
Pour 1 can of enchilada sauce into a wide bottomed bowl or pie tin. Dip the tortillas, one at a time, into the sauce to lightly coat. Spoon the halibut in equal portions into the center of each tortilla. Make the enchiladas by tightly rolling the tortillas and place 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 aluminum foil
Bake in preheated oven for 45 minutes. Top with avocado slices to serve.