Asafetida (also called hing) is a spice with a strong, distinctive odor; a tiny amount adds a lot of flavor to a dish. It’s extremely expensive in its pure form so it’s generally compounded with wheat or rice flour and turmeric. In my view it’s essential for making this dal. While quality varies in commercial brands, I have been impressed with the asafetida from Pure Indian Foods. Incidentally, the spice is thought by some to have anti-flatulence qualities and is added to many of the legume dishes in India.