Flake fish (and cook if not already cooked).

Make white sauce with butter, flour, salt, pepper and milk. Stir until thickened. Add onion and dry mustard.

Arrange fish in bottom of dish or in individual servings.

Cover fish with cream sauce and top with grated cheese.

Place casserole(s) in shallow pan of water and bake at 350 degrees F for 30 minutes.