In 3 1/2 or 4-quart slow cooker, combine all ingredients except tomatoes, spinach and yogurt; mix well.
Cover; cook on Low setting for 8 to 9 hours.
About 5 minutes before serving, stir tomatoes into soup. Cover; cook an additional 5 minutes or until thoroughly heated.
Just before serving, remove and discard bay leaves. Top individual servings with 1/4 cup spinach and 1 tablespoon yogurt.