Preheat oven to 180 degrees C. Spray a baking dish with cooking spray.
Stir together water, vinegar and salt. Add the fish fillets then bring to a boil. Boil until fish flakes easily with a fork. Set in the greased baking dish.
In a saucepan melt butter, then stir in flour with a fork. Whisk in milk then whisk in salt and cayenne. Bring to a boil and stir until thickened. Remove from heat and stir in lemon juice and tasty cheese. Pour over fish.
Sprinkle breadcrumbs and Parmesan over the top then bake in preheated oven until golden brown, about 15 minutes.
If you don't have tarakihi, you can use any white fish, including hoki, cod, snapper or gurnard. I just prefer tarakihi for this recipe.