Put the meat to boil in cold water on medium-low heat. Skim the foam and let it simmer for 30-45 minutes. Add 1 tbsp salt and a pinch of black peppercorns.
While the chicken is cooking, prepare the veggies. Wash and peel the carrots, parsnips, root celery and root parsley. Cut all the root vegetables in large chunks. Cut the onions in half. Add them to the meat to cook together until the meat falls off the bones. That will take another 30-40 minutes.
Once the meat and vegetables are cooked, turn off the heat and take the veggies out on a large plate to cool a little so they can be chopped in small cubes. You may want to use only 3/4 of them. Ditch the onions. Take out of the fridge the sour cream and eggs to bring them to room temperature.
In a large bawl, take off the chicken and try to move quickly so the meat remain hydrated. Strain the soup to remove any particles left and place it back to the same pot. Then put 2-3 ladles of soup in the meat bawl while you work on shredding it with 2 forks. Get rid of the bones and skin.
Put the shredded meat and the chopped root vegetables back into the soup. Turn on the heat again and bring to boiling point for about 3-4 minutes so the flavors blend. Then tun off the heat and let the soup cool for about 10 minutes before you add the cream mixture.
Mix the sour cream and the egg yolks into a bawl. Gently add to this mixture 2-3 ladles (one at a time) of soup to dilute and temper the sour cream before adding it to the hot pot of soup. Whisk well after each addition to incorporate the liquid into the mixture.
Finally, add the sour cream mixture to the pot stirring continuously until well blended. Add the vinegar and grated garlic. Sprinkle with freshly chopped parsley and serve.