In a 6-qt. slow cooker, combine first nine ingredients. Cook, covered, on low until chicken is tender, 6-8 hours., For croutons, cut Texas toast into cubes; bake according to package directions. Add spinach to soup; heat through, stirring occasionally. Top servings with cheese and warm croutons. Freeze option: Freeze cooled soup in freezer containers. To use, partially thaw in refrigerator overnight. Heat through in a saucepan, stirring occasionally. Prepare croutons as directed. Top soup with cheese and croutons.