Place the chicken breasts in the slow cooker, followed by the chopped onion, sliced carrots, celery, minced garlic, thyme, rosemary, and bay leaf. Generously season the mixture with kosher salt and freshly ground black pepper. Pour the chicken broth over the top, ensuring all ingredients are submerged.
Cover the slow cooker and cook on the Low setting for 6 to 8 hours, until the chicken is tender and easily shredded. Once done, remove the chicken and shred it using two forks. Discard the thyme, rosemary, and bay leaf.
Return the shredded chicken to the slow cooker and stir in the egg noodles. Cover and cook on Low for an additional 20 to 30 minutes, or until the noodles are just tender. Serve warm.