Place all ingredients in a piece of cheesecloth and tie with twine. Set aside.
Melt the butter in a large saucepan over medium-high heat. Add the sorrel and onion and cook for 1 or 2 minutes, stirring constantly. Shake the flour over the sorrel mixture and cook for 2 minutes longer, stirring occasionally. In a separate saucepan, bring the stock to a boil, then add the sorrel mixture. Return to a boil and reduce the heat to a simmer. Add the bouquet garni, bread crumbs, tarragon, salt, and pepper and simmer the soup for 30 minutes to an hour or until the liquid is reduced and the sorrel is cooked through. Remove the bouquet garni. Transfer the soup to a food processor or blender and puree until smooth. Return to a clean saucepan, stir in the cream and bring just to a simmer; do not boil. Ladle the soup into warm serving plates.