I have used oil for roasting the spices and for tempering. Any oil can be used, coconut oil also lends an amazing flavor. If you are ok with non vegan version, use ghee and the flavors would be amazing too!
Heat oil in a pan, add the ingredients listed under the title 'For Roasting', add the hemp seeds at the last. Roast till golden. set aside to cool.
Add the ground mixture to the simmering rasam, followed by cooked dal, salt, and water. Mix and when it begins to boil, remove from the flame.
Add the tempering to the rasam. Add the herbs as garnish.