Chop vegetables: Chop the onion. Halve the leek white lengthwise and slice the halves thinly. Chop the celery/celeriac, carrot, and potatoes into small cubes.
Sauté: Melt butter in the pot. Add vegetables and cook for about 5 minutes, adding a splash of water halfway through. Stir often. The vegetables should become softer but not take any color.
Simmer: Add bay leaves, thyme, rosemary, stock and milk. Cover with a lid, leaving a crack open, bring to a boil, and simmer softly for about 10 minutes or until the vegetables are soft.
Blend the soup (Note 4). Return to the stove and add the shredded turkey. Simmer until the turkey meat is heated through, about 5 minutes.
Adjust the taste with nutmeg, salt, and white pepper.
Thicken the soup: In the meantime, whisk heavy cream and egg yolk in a small bowl. Whisk this mixture into the soup. Don't let the soup come to a boil anymore.
Adjust: Remove from heat, adjust the taste again, and stir in the parsley. Sprinkle with a few red peppercorns if desired. Serve immediately.