Comments: In Iran, there are as many as five different types of rice (berenj). Since Iranian varieties are not available in the United States, use basmati rice as an alternative. Boiled and then steamed, Persian rice combines fluffy grains with a crunchy top layer known as Tah dig, a crucial element of the dish, which forms at the bottom of the rice pot as the dish cooks. For perfect, crisp and golden-brown Tah dig, use a large, nonstick pot to cook the rice. To further prevent the crust from sticking to the pot, line the bottom with aluminum foil. Just before serving, invert the finished dish onto a plate so that the crust is on top.