Season the boneless, skinless chicken breasts generously with kosher salt and freshly ground black pepper. Place them in the bottom of a 6-quart slow cooker.

Pour in the chicken stock, followed by the diced onion, chopped carrots, and celery. Mince the garlic and add it to the mix, along with dried thyme, dried rosemary, and bay leaves.

Cover the slow cooker and set it on low heat for 6 to 8 hours.

Once the cooking time is up, carefully remove the chicken and shred it into bite-sized pieces. Return the shredded chicken to the slow cooker.

Break the spaghetti into thirds and add it to the slow cooker, stirring gently to distribute it evenly.

Once the pasta is tender, stir in the freshly squeezed lemon juice and chopped fresh parsley. Adjust seasoning as needed.

Scoop the soup into warm bowls, garnishing with additional parsley if desired.