Peel and slice the carrots into 1/4-inch rounds. Slice the celery (including leaves, if using). Dice the onion into small, even pieces. Mince the garlic. Try to keep the vegetable pieces roughly the same size so they cook evenly.

Add the carrots, celery, onion, and garlic to the bottom of a 5–6 quart slow cooker. Place the boneless, skinless chicken breasts or thighs on top of the vegetables so the chicken poaches gently in the broth.

Pour in the low-sodium chicken broth. Add the bay leaf, dried thyme, dried parsley, dried oregano, kosher salt, black pepper, and olive oil or melted butter (if using). Gently stir the broth and seasonings without disturbing the chicken too much.

Cover the slow cooker and cook on LOW for 6–7 hours or on HIGH for 3–4 hours, until the chicken is fully cooked through and shreds easily with a fork. Cooking on LOW gives the best flavor and texture, but HIGH works if you are short on time.

When the chicken is done, remove it from the crockpot to a cutting board or large bowl. Let it cool briefly, then shred it using two forks (or use a stand mixer with a paddle attachment on low speed for 20–30 seconds). Discard any large fat pieces if desired.

If using uncooked egg noodles or small pasta, stir them directly into the hot soup, cover, and cook on HIGH for 15–20 minutes or until just tender. If using cooked rice instead, stir it into the soup at the end and let it warm through for 5–10 minutes. Avoid overcooking noodles, as they can become too soft if left in the slow cooker too long.

Add the shredded chicken back into the crockpot and stir well to combine with the vegetables, broth, and any noodles or rice. Taste and adjust seasoning with additional salt, pepper, or dried herbs as needed.

Stir in the fresh lemon juice to brighten the flavors. Sprinkle in chopped fresh parsley and stir again. Remove and discard the bay leaf before serving.

Ladle the hot chicken soup into bowls and serve immediately. Enjoy as is, or pair with crusty bread, crackers, or a grilled cheese sandwich.