Dice the leftover roasted chicken into bite-sized pieces and set aside.
Slice the carrots thinly and chop the celery and onion into small, uniform pieces for quick, even cooking.
Add the shredded chicken, sliced carrots, chopped celery, and diced onion to the slow cooker.
Pour the chicken broth over the ingredients, making sure everything is just covered, then sprinkle in the dried thyme and a pinch of salt and pepper.
Cover the slow cooker with the lid and set it to low. Let it simmer for 6 to 8 hours until the vegetables are tender and the flavors meld, filling your kitchen with a warm aroma.
Check the soup around the 6-hour mark: vegetables should be soft, the chicken tender and easily shredded, and the broth fragrant and flavorful.
If the soup is too thick, stir in a little hot water or additional broth to loosen it up. If it’s too watery, uncover and cook on high for 30 minutes to thicken slightly.
Taste the soup and adjust the seasoning with more salt, pepper, or a squeeze of lemon for brightness. Stir in chopped fresh parsley just before serving for a fresh, herbal note.
Turn off the slow cooker and let the soup sit for 5-10 minutes to settle the flavors. Then, ladle into bowls, garnish with additional parsley if desired, and enjoy a warm, comforting bowl.