Originally, this dish was made with smoked or salted fish, but it can be made with any good solid fresh or frozen white fish. I use boneless, skinless cod, but you can use haddock.
You can loosely cover the pan with foil for part of the baking time, but remove the foil for the last 15 minutes.
Do not skimp on the milk, and do not bake in too hot of an oven. If you don't add enough milk, it will evaporate during cooking. If the oven is too hot, the milk will curdle.