Plunge the sorrel into salted boiling water and drain the moment the water returns to a boil and all the leaves have turned grayish. Put to stew gently in half the butter, stirring occasionally, until excess liquid has evaporated and the leaves have fallen into a complete purée—about 20 minutes.
Stew the onions, salted, in the remaining butter for at least ½ hour, covered, stirring or tossing regularly. The saucepan should be heavy and not so large as to impose a depth of less than
Mix together the sorrel and the onions in a bowl and, when no more than tepid, whisk or beat the cream, eggs, and seasonings apart; then pour the mixture slowly into the onions and sorrel, stirring all the while. Pour into the pastry shell and bake in a 375° oven for about 40 minutes or until swelled, firm in the center, and lightly colored. Serve warm 20 minutes to ½ hour after removing from the oven. A well-chilled, young, light, dry white wine with a suspicion of sparkle or a light-bodied Champagne are perfect companions.