In large pan, melt butter and stir in onion, cooking gently on low heat just until onion is golden--be careful not to burn it! Stir in flour, increase heat to medium, and whisk for a minute or two. Add diced tomatoes, cook on medium until mixture is thickened. Pour in soup, milk and tomato juice and heat, but do not boil. Season to taste. Serve immediately, or pour into crock-pot and heat on low until soup is needed.