First cut up the chicken into individual pieces, you won't need the breasts for this recipe. Next, clean the meat - this step is optional but it makes the soup less cloudy. The best way to do it is to pour water into a large pot, add the meat, clean it thoroughly from all the blood residues and debris, then carefully discard the water, leaving the meat in the pot.
Add herbs and spices to the pot, but they should be under the meat (otherwise they float the surface and make skimming of the 'scum' harder).
Prepare the vegetables: peel the carrots, celery root, and celeriac, cut in half. Peel the onion and burn it over a flame or place cut-side down in a dry pan and cook until dark brown/black. Cut off the dark green part of the leek, use only the white and light green part, cut in half, clean thoroughly.
Add the vegetables to the pot.
Add cold water to the pot. There should be about 2 inches (4-5 cm) of water above the meat and vegetables.
Bring the soup to a gentle boil over medium heat. When it comes to the boil reduce the heat immediately and cook uncovered over minimum heat for 6 hours (or a minimum of 3 hours, if cooking the soup with beef I recommend cooking it for 4 hours, not 6 hours). Do not add salt to the soup (we're salting the soup at the end). It's super important that the soup is only SIMMERING - this means you should see a couple of small bubbles on the surface of the water. The soup shouldn't boil because it will make it cloudy. If you've noticed that your soup is boiling it will be cloudy but after some cooking time, it should be clear again.
When the soup comes to a boil you will see foam form on the surface. This is just coagulated protein and doesn't affect the flavor of the soup but it can make it cloudy so I skim it with a small sieve or a spoon. Skim just the biggest pieces of scum and foam, you don't have to do it very thoroughly. Repeat this a couple of times while the soup is simmering.
When the time is up, carefully remove any scum and foam that is on the surface of the soup, then strain the soup through a fine-mesh strainer. Discard the vegetables. When the meat is cool enough to handle, pull it off the bones and tear it into shreds with your fingers.
Season the soup with salt to taste. Place some pasta in bowls and pour in the soup. Season the soup with ground black pepper to taste and enjoy!