You have a good base,cook fish gently in milk and keep milk to make a sauce,put some butter/margarine in a pan add flour and cook the roux out,add the milk from the fish and you will maybe need more milk until the sauce thickens,break up fish into chunks and then put in dish add chopped parsley and peas if liked also some quartered hard boiled eggs,you could add cheese to the sauce,but i would pour over the sauce mixing gently,cool and then cover in mash fork lightly and sprinkle with cheese and breadcrumbs,forgot to say season sauce with salt and pepper too taste bon appetie!