Preheat oven to 190 degrees C. Grease a baking dish.
In a saucepan melt butter and saute the onion. When the onion is soft blend in the flour. Whisk in the milk then add the bay. Bring to a boil then remove from heat. Remove bay leaf, stir in cheese then season to taste with salt and pepper.
Stir in the tarakihi then pour into prepared baking dish. Sprinkle with extra cheese and breadcrumbs. Bake in preheated oven until the top has browned and fish is cooked though, about 20 minutes.
You can use hoki, cod or snapper in place of the tarakihi.