Place the chicken breasts into the slow cooker, spreading them out evenly.
Add the diced carrots and celery around the chicken, then pour in the chicken broth to cover everything.
Sprinkle in the fresh thyme, and season with salt and pepper to taste. Cover the slow cooker and set to low for 6 to 8 hours, letting the flavors meld and the chicken become tender.
Once the cooking time is up, carefully remove the chicken breasts, shred them with two forks, and set aside.
Turn the slow cooker to high, add the egg noodles directly into the hot broth, and stir. Cover and cook for 10 to 12 minutes, until the noodles are chewy and tender.
Return the shredded chicken to the pot, stirring to combine. Squeeze in the lemon juice if using, and taste for seasoning, adding more salt and pepper if needed.
Once everything is heated through and the flavors have melded, turn off the slow cooker and let the soup sit for a few minutes.
Ladle the soup into bowls, garnish with chopped parsley, and serve hot for a cozy, comforting meal.