A crisp edge is essential to the appeal of tah-dig, a Persian dish in which the bottom layer forms a golden crust (basmati rice is ideal because it turns crunchy quickly); ours is topped with a layer of perfectly browned potatoes.Dried cherries and luscious threads of duck confit are steamed within the cinnamon-scented rice layer. Long-grain basmati, famed for its perfumed aroma and fluffy, delicate texture, is the quintessential rice of Middle Eastern and Indian cookery.  On "The Martha Stewart Show," Martha prepares this recipe without the duck confit.