Preheat oven to 175°C/350°F. Peel sweet potatoes, chop into small cubes. Grease a baking dish with butter and transfer the sweet potato cubes to the baking dish. Season with salt.
Pour white wine over the sweet potatoes. Bake at 175°C/350°F for approx. 20 min.
Pluck parsley leaves and chop finely. Season both sides of the fish fillets with salt. Remove sweet potatos from the oven and place the fish on top. Sprinkle with parsley and distribute butter onto the fish fillets. Return the baking dish to the oven and bake for approx. 5 min., or until fish is cooked to your liking. Serve with parsley and enjoy!