In a large, lined slow cooker, stir together the beans, chicken broth, onion, carrots, celery, garlic, thyme, bay leaf and Parmesan rind, if using. Cover and cook on high heat until the beans are nearly tender, about 3 1/2 hours.
Place the sausage meatballs on top of the beans. Cover; cook until the meatballs are no longer pink in the center, about 25 minutes.
Discard the thyme sprigs, bay leaf and Parmesan rind. Stir in the spinach, then cover and let stand until the spinach wilts, 10 minutes.
Stir in the lemon juice; season. Serve with crusty bread.