Sorrel Tart

• 10 ounces sorrel, stems pulled off backwards, washed in several waters, drained
• Salt
• ¼ cup butter

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 t