In a saucepan or skillet, melt butter over medium heat. Whisk in flour, stirring constantly, until a smooth paste is formed. Gradually add milk and reserved liquid from peas, stirring constantly, until a smooth thick gravy is formed.
Flake salmon into a bowl, breaking apart any large pieces. Stir salmon and peas into the sauce carefully with a wooden spoon to avoid mashing the peas. Cook until heated through. Adjust seasoning with salt and pepper.
Toast bread in toaster oven or broiler pan. Butter, if desired, and top with salmon mixture.
