Add the onion, carrots, celery, garlic, zucchini, kidney beans, and diced tomatoes to the slow cooker.
Pour in the vegetable broth and stir in the dried basil, dried oregano, salt, and black pepper.
Cover and cook on low for 6 hours until the vegetables are soft.
Stir in the pasta shells, cover again, and cook for 30 more minutes until the pasta is tender.
Ladle the soup into bowls and top each serving with a few fresh basil leaves.