I chopped the lemongrass. You can crush slightly too, for getting the most of it's flavour.
Place lemongrass, tomato, green chilli, 2 tablespoon coriander leaves in a vessel. Add 2 cups water to it, along with turmeric (I used fresh turmeric, grated).
Bring to boil. Simmer for 2 minutes. Add crushed pepper powder.
Dissolve cooked dal and make total one cup with water. Pour into the boiling rasam. Add enough salt and mix well.
Switch off the flame once the rasam starts to boil frothy. Squeeze lemon.
Temper with the items given under to temper.
Garnish with remaining coriander leaves, kaffir lime leaves if adding.