In a saucepan, combine tomatoes, celery and onion; bring to a boil. Reduce heat; cover and simmer for 15 minutes, stirring often. Cool for 10 minutes; pour into a blender. Cover and process until smooth. In a large saucepan, heat oil; stir in flour until smooth. Gradually add milk; bring to a boil. Cook and stir for 2 minutes. Gradually stir in tomato mixture. Add salt if desired and pepper; heat through. Top individual servings with sour cream and parsley.