Place chicken breasts in the bottom of a 6-quart slow cooker and drizzle with olive oil.
Add chopped yellow onion, sliced celery, and sliced carrots around the chicken in the slow cooker.
Pour chicken broth over the vegetables and chicken, then add dried thyme, bay leaf, salt, and black pepper, stirring gently to combine.
Cover and cook on LOW for 6-7 hours or HIGH for 3-4 hours, until chicken is tender and easily shreds with a fork.
Remove chicken from slow cooker and shred into bite-sized pieces using two forks, then return shredded chicken to the broth.
Stir in egg noodles, cover, and cook on HIGH for 20-30 minutes until noodles are tender but still firm to the bite.
Turn off slow cooker, remove bay leaf, and stir in fresh parsley just before serving.
