In a 4 to 5 quart slow cooker, combine the broth, water, celery, carrots, lentils, ham, onion and thyme. Cover and cook on the low-heat setting for 7 to 8 hours or on the high-heat setting for 3½ to 4 hours.
When ready to serve, stir in the spinach. Ladle the soup into bowls and top with Parmesan cheese, if using.
If you're preparing this recipe as Dairy Free, leave out the Parmesan cheese.
If you're preparing this recipe as Gluten-Free, make sure the ham that you use does not have any added gluten, and be sure to use a brand of chicken broth that is designated as GF.
Nutritional information does not include optional cheese.