Melt the butter in a saucepan over medium heat. Whisk in the flour and cook, whisking, until the mixture is smooth and beginning to brown. Whisk in the fish stock, bring to a simmer, and simmer, stirring until thickened. Add optional ingredients as you please. You can pour the sauce directly onto any fish, right there on the plate, or you can create a casserole by pouring the sauce over some bland cooked fish. Add boiled sliced potatoes, if you wish, and heat at 350°F for 20 minutes.
If you have the time, overcooked fish can also be made into fish cakes or croquettes. The binding sauce will restore some of the moisture.
