Asafetida (a.k.a. asafoetida or hing) is an oniony seasoning that is used in small quantities in Indian recipes. It is known to aid in digestion and reduce gas, which is probably why it's commonly employed when cooking lentils and other vegetable dishes.

It isn't very easy to find asafetida in western groceries, although if you shop at a store with a large spice collection, you might check for it. Otherwise, you should look in an Indian market or specialty gourmet-type shop.