SWEETBREADS (Riz de Veau) The thymus gland of the calf is a great delicacy in France.  It must always be precooked as follows (so that any sweetbread recipe inevitably must be 2-Step Cookery):Blanch sweetbreads by covering with cold water, adding 2 T white vinegar and bring to a boil. Reduce heat sharply and simmer for 20 minutes. Then drain, and plunge the sweetbreads into icy-cold water (sacrifice a tray of ice cubes, if you must, to pack about the sweetbreads). Let stand until cool enough to handle; trim away any tubes or membranes, slice sweetbreads in half—or in thick slices—depending on the recipe ahead. Wrap carefully and store overnight (but no longer) in the refrigerator. Frozen sweetbreads exist in the West; in the East, where French gastronomy is more general, sweetbreads are only to be had from first-class meat markets at a shocking price—but worth it, of course, if you favor la haute cuisine.