In a small saucepan melt butter over medium low heat. Whisk in flour and salt to make a paste. Gradually whisk in milk. Stirring constantly cook until sauce is thick enough to coat the back of a spoon.
Select a baking dish large enough to accommodate all the fillets in one layer. Coat with cooking spray. Arrange fish in the dish and cover with prawns.
Pour white sauce over prawns. Sprinkle with grated cheese.