By Harold McGee
An ancient way of cooking fish is to enclose it in a layer of some material—clay, coarse salt, leaves—to shield it from direct heat, and then cook the whole package (see box). The fish inside will be more evenly and gently cooked, though checking the temperature is still essential to avoid overcooking. Showy preparations with an edible crust of pastry or brioche (French en croûte) are baked in the oven. A more versatile technique is the use of a thin envelope of parchment (en papillote), or aluminum foil, or a leaf, either neutral (lettuce) or flavorful (cabbage, fig, banana, lotus, hoja santa), which can be used with almost any heat source, from grill to steamer. But once the contents get hot enough, nearly all the heating is done by the juices of the fish and vegetables themselves, which surround the food and steam it. The envelope can be served intact and opened by the diner, releasing aromas that would otherwise have been left behind in the kitchen.