Partially cook bacon in a large Dutch oven over medium heat until slightly browned; add onion, and cook until onion is tender and bacon is crisp. Add bluefish and cook, stirring occasionally, 4 to 5 minutes or until fish begins to brown. Add all except butter, flour, and evaporated milk, and simmer 20 minutes or until potatoes are tender. Melt 3 tablespoons butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly. Stir white sauce into fish mixture; simmer, stirring occasionally, for 20 minutes or until thickened.