Peel and cut the carrots into half-moons.
Finely dice the celery sticks and onion.
Wash and peel the potato. Leave the potatoes whole, do not cut them.
Finely chop the celery leaves.
Add the olive oil into the pot and heat over high heat. Add the black pepper and fry for 1 minute or until the black pepper starts to crackle.
Reduce heat to medium. Add the onion, celery, half of celery leaves, and carrots and Sauté for 5-6 minutes or until soft.
Add the whole potatoes, lemon wedge and thyme.
Pour the stock into the pot with the vegetables and add another 6 cups of fresh water.
Bring to a boil and then reduce heat to low. Cook until the potatoes are tender.
Remove the potatoes with some of the broth.
Purée using an immersion blender. Return the potato mix to the pot along with the cooked chicken pieces.
Turn off the stove.
In a small bowl, mix the yogurt with egg yolk and the juice of half a lemon. Stir well.
Temper the egg mix by gently pouring a spoonful of the soup stock in a thin stream into the egg mixture mixing while you add. Repeat the step until the temperature of the egg mixture and the remaining soup are balanced.
After you have done this step, pour the resulting slurry into the soup. Stir well to combine.
Sprinkle with celery or parsley leaves (your choice) and serve.