Preheat oven to 325 degrees F (165 degrees C).

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 pan large enough to accommodate all the fillets in one layer. Coat with cooking spray. Arrange fish in pan, and cover with shrimp.. Pour white sauce over shrimp. Sprinkle with grated cheese.

Bake, uncovered, for 20 to 25 minutes.