Put tomato soup, salt if you wish, and butter in slow cooker. Blend well. Cover and cook on high for 1 hour. Meanwhile, place flour and 1 cup milk in 2 quart microwave safe container. Whisk together until big lumps disappear. Then whisk in remaining milk until only small lumps remain. Place flour-milk mixture in microwave and cook on high for 3 minutes. Remove and stir until smooth. Return to microwave and cook on high for another 3 minutes. Add thickened milk slowly to hot soup in slow cooker. Heat thoroughly for 10 to 15 minutes.
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