Place the chicken breasts, marinara sauce, chicken broth, diced onion, minced garlic, oregano, basil, salt, and pepper into a 6-quart slow cooker.
Cover and cook on LOW for 6 hours (or HIGH for 3 to 4 hours).
Remove the chicken from the slow cooker and shred it using two forks. Return the shredded chicken to the slow cooker.
Stir in the uncooked pasta. Cover and cook on HIGH for 20 to 30 minutes until the pasta is tender.
Stir in the grated Parmesan cheese until fully incorporated.
Ladle the soup into bowls and top immediately with shredded mozzarella cheese and fresh parsley.