Season the halibut with garlic, salt and pepper; cook in microwave until slightly cooked, about 2 minutes. Mix together the spring onion, green pepper, fresh coriander, soured cream, mayonnaise and 120g cheese in a bowl. Gently fold the halibut into the mixture.
Lightly grease a large oven-proof baking dish. Pour 1/2 of enchilada sauce over the bottom of the baking dish.
Pour the rest of the sauce into a bowl. Dip the tortillas, one at a time, into the sauce to lightly coat. Spoon the halibut in equal portions into the centre 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 remaining cheese. Cover the dish with foil.
Bake for 45 minutes. Top with avocado slices to serve.