Place the chicken breasts at the bottom of the slow cooker and pour in the chicken broth, ensuring the chicken is mostly submerged. Cover and cook on low for 4-6 hours until the chicken is tender and easily shredded.
While the chicken cooks, prep the vegetables by peeling and slicing the carrots and celery thinly, and chopping the onion and garlic finely. This helps them soften quickly and release flavor.
Once the chicken is cooked, remove it from the slow cooker and set aside to cool slightly. Shred the chicken into bite-sized pieces using two forks, then return it to the broth.
Add the sliced carrots, celery, chopped onion, and minced garlic to the slow cooker. Stir well and cover, cooking on high for about 30-45 minutes until the vegetables are tender and fragrant.
Check the soup; taste the broth and adjust seasoning with salt and pepper as needed. Stir in the thyme and let everything simmer for a few more minutes to meld flavors.
Add the egg noodles directly into the slow cooker. Cover and cook on high for 8-10 minutes until the noodles are tender but not mushy, stirring halfway through if needed.
Taste the soup once more; squeeze in lemon juice if using, and adjust salt and pepper for flavor balance. Stir in chopped fresh parsley for brightness.
Turn off the slow cooker and let the soup rest for 5 minutes. This allows flavors to settle and noodles to finish softening perfectly.
Use a ladle to serve the hot, fragrant soup into bowls, making sure each has a good mix of chicken, vegetables, and noodles. Garnish with extra parsley if desired and enjoy the cozy, comforting meal.
