Preheat oven ton 200 degrees C. Grease a baking dish.
In the baking dish add the mushrooms, onions and tarakihi fillets. Pour in the wine then squeeze over the juice from the lemon. Season with salt and white pepper, cover then bake in the preheated oven until fish flakes easily with a fork, about 20 minutes.
In a saucepan melt the butter then stir in the flour. When the fish is cooked remove it to a plate then pour all the cooking juices into the saucepan and whisk together with the butter flour mixture. Simmer until thickened, about 5 minutes.
Serve fish drizzled with the sauce. Goes great with potatoes.
Tarakihi is a fish that is consumed in New Zealand. You can use hoki, cod, snapper or any white fish fillets.