Heat the butter in a 3- or 4-cup saucepan and add the onion. Cook, stirring with a wire whisk, until the onion is wilted. Do not brown the onion.
Sprinkle the onion with the flour and continue cooking and stirring briefly until blended. Add the milk gradually, stirring with the whisk. When blended and smooth add the cream and stir. When thickened and smooth add salt and pepper to taste. Simmer for 3 minutes, stirring occasionally.
Drain the cans of tuna or salmon and empty the fish into a mixing bowl. Flake with a fork but leave bite-sized chunks. Add the chunks to the sauce. Heat thoroughly. Serve on toast, if desired, or with rice.
Cook ½cup finely minced celery and 2tablespoons finely minced green pepper with 1tablespoon butter in a small skillet. Cook just until celery is crisp-tender. Add this to the above sauce along with the tuna.