Peel the carrot, turnips, potatoes, and celery stalk. Trim the leek whites, then dice all these vegetables into even pieces.
Chop the sorrel and onion. Melt the butter in a pan and quickly sauté the sorrel and onion.
Bring the defatted chicken stock to a boil in a saucepan. Add the diced vegetables and simmer for 15 minutes. Reserve 2 tablespoons of the cooked diced vegetables for garnish, then purée the rest with the sorrel and onion.
Return the purée to the saucepan. Season with salt and freshly ground pepper, then add the reserved diced vegetables.
In a bowl, whisk the egg yolk with the heavy cream.
Remove the pan from the heat and gradually whisk in the cream mixture for a silky finish.
Serve in a soup tureen or bowls, garnished with finely chopped chervil and chives. Enjoy hot or chilled.
