Place the chicken thighs in the large pot and add enough water to cover them. Bring to a boil, then reduce the heat and simmer for about 30 minutes until the chicken is cooked through and tender. Skim off any foam that forms on the surface.
Carefully remove the chicken from the broth and set aside to cool slightly. Once cool enough to handle, shred the meat into bite-sized pieces using two forks.
Meanwhile, heat olive oil in the same pot over medium heat. Add chopped carrots and celery, sautéing until they begin to soften and release their aroma, about 7 minutes. Stir occasionally to prevent sticking.
Pour in the chicken stock and bring the mixture to a gentle simmer. Add the shredded chicken back into the pot along with dried thyme. Let it simmer for another 10 minutes, allowing flavors to meld and the broth to deepen.
Add chopped fresh parsley and the zest of the lemon to the soup. Squeeze in the lemon juice, then season with salt and pepper to taste. Stir well to combine all the bright, fresh flavors.
Taste the broth and adjust seasoning if needed. Serve the hot soup in bowls, garnished with extra parsley or lemon slices if desired. Enjoy the comforting warmth and fresh citrus notes in every spoonful.
