This is Julia Childs’s recipe, just adapted a bit for simplicity. It makes six servings.
1/3 cup minced green or yellow onions
4 tablespoons butter, divided
3 to 4 cups packed fresh sorrel leaves, washed, dried, and cut into thin strips
1/2 teaspoon salt
3 tablespoons flour
5 and 1/2 cups boiling chicken stock
2 egg yolks
1/2 cup heavy whipping cream
Warm up 3 tablespoons of the butter in a large, heavy-bottomed saucepan over medium heat. Sauté the onions for 5-10 minutes, or until tender and translucent. Stir in all but a handful of the sorrel and the salt, cover the pot, and turn the heat down to low. Cook until the leaves are tender and wilted, about 5 minutes.
Now turn the heat back up to medium and sprinkle in the flour. Cook for three minutes, then pour in the boiling stock, stirring well. Simmer for 5 minutes.
Meanwhile, blend the yolks and cream in a large mixing bowl. Slowly, drop by drop, beat in a cupful of hot soup. Gradually beat in the rest, pouring it in a thin stream as you beat. Return the soup to the saucepan and cook, stirring constantly, for 1-2 minutes over medium heat. Do not bring the soup to a simmer—you just want to cook the eggs. Turn off the heat and stir in the remaining tablespoon of butter.
For hot soup, serve immediately, garnished with the remaining sorrel leaves. For cold soup, leave out the final tablespoon of butter and chill before serving.