Peel the carrots and celery and cut them into large chunks. Peel and quarter the onion. Peel the garlic cloves, halve the leek, wash it thoroughly, and also cut it into large chunks.

Place the chicken thighs in a large pot. Add the carrots, onions, garlic, celery, and leeks, then cover with water. Add salt, bay leaves, peppercorns, and parsley, and bring to a boil.

Once the water boils, reduce the heat slightly and let the soup simmer gently, uncovered, for 90 minutes. In the meantime, occasionally skim off and discard the foam that will repeatedly form on the surface using a spoon or spatula.

Remove the meat from the pot and place it on a large board or baking sheet. Strain the soup through a fine sieve, collecting the broth in a second pot. Discard any overcooked vegetables and return the broth to the stove. Add the soup vegetables and cook them in the gently simmering broth until tender but still firm to the bite.

Remove the skin from the chicken thighs and discard it. Separate the meat from the bone and remove any cartilage. Shred the meat into bite-sized pieces using two forks and return it to the soup.

Season the soup again with salt and pepper and serve with soup noodles and fresh parsley (if desired).