Preheat oven to 200 degrees C. Lightly grease a 20x30cm baking dish and set aside.
In a saucepan over medium heat, melt butter and saute onion, prawns and mushrooms until onion is tender. Stir in crab meat, salt, pepper and paprika; heat through.
Spoon the prawns and crab mixture onto each flounder fillet. Roll the fillet up and fasten with toothpicks. Place rolled fillets in prepared baking dish.
In a small bowl, whisk together soup, chicken stock and water until smooth. Pour over the fillets. Sprinkle cheese over the top.
Cover the baking dish with aluminium foil and bake in preheated oven for 30 minutes. Remove from oven and remove foil. Sprinkle the parsley over the top of the fillets. Return the dish to the oven and bake uncovered for an additional 5 minutes or until fish flakes easily with a fork.