Brown the Italian sausage in a skillet over medium heat, breaking it into small pieces as it cooks. Drain any excess fat.
Add the cooked sausage to the slow cooker along with the onion, carrots, celery, garlic, cannellini beans, chicken broth, Italian seasoning, salt, and black pepper.
Stir to combine, then cover and cook on low for 6 to 7 hours until the vegetables are soft.
Stir in the baby spinach and let it wilt for 5 minutes before serving.
Ladle the soup into bowls and top each serving with grated Parmesan cheese.