Finely mince the yellow onions.

In a covered pan, cook the minced onions in 6 tablespoons of butter over medium-low heat for 10 minutes, ensuring they do not brown.

Wash and dry the sorrel leaves, then cut them into thin strips.

Add the sorrel strips and 1 teaspoon of salt to the pan with the onions.

Stir the mixture over low heat for about 5 minutes, allowing the sorrel to wilt.

Sprinkle 5 tablespoons of flour over the sorrel and onion mixture.

Stir continuously, cooking over moderate heat for 5 minutes to create a roux.

Gradually stir in 12 cups of boiling homemade chicken stock to the pan, ensuring there are no lumps.

Simmer the soup for 10 minutes, allowing the flavors to meld.

Season the soup with salt and pepper to taste.

In a separate bowl, lightly beat 4 egg yolks with 1-1/2 cups of heavy cream.

Pour 1 cup of the hot soup into the egg yolk mixture, tempering it to prevent curdling.

Slowly add the tempered egg yolk mixture back into the soup pot.

Stir gently and heat through, being careful not to let the soup boil after adding the egg yolks to prevent curdling.

Serve the soup hot, enriching each serving with 3 tablespoons of softened butter at the last minute.

If serving the soup cold, omit the butter.