Brown the Italian sausage in a skillet over medium heat, breaking it into small pieces as it cooks. Drain excess fat and transfer the sausage to the slow cooker.
Add the diced onion, sliced carrots, sliced celery, and minced garlic to the slow cooker.
Stir in the drained white beans, chicken broth, Italian seasoning, salt, and black pepper.
Cover and cook on low for 6 hours or on high for 3 hours, until the vegetables are tender.
Stir in the fresh spinach during the last 10 minutes of cooking so it wilts into the soup.
Ladle into bowls and top each serving with grated Parmesan cheese.