This soup can be served hot or cold.
In a large saucepan, melt 3 tablespoons butter over medium heat. Add onion and cook, without browning, until softened, 6 to 8 minutes. Add sorrel and season with 1/2 teaspoon salt. Stir over low heat for about 5 minutes.
Sprinkle in flour and cook, stirring, over moderate heat for 5 minutes. Stir in boiling stock. Simmer for 10 minutes. Season with salt and pepper.
In a small bowl, beat yolks lightly with cream. Whisking constantly, slowly pour 1 cup hot stock mixture into egg mixture, then return back to saucepan. Do not let soup boil after egg yolks have been added or yolks will curdle.
Stir in softened butter and serve hot with a dollop of creme fraiche and chervil. If serving the soup chilled, do not add the softened butter.