Wash the tur dal and cook it in a pressure cooker with a little turmeric powder. It should be soft and well cooked.
Take water in a pan, add tomato pieces, grated ginger, slit green chilli, turmeric powder, coriander and curry leaves to it. Boil them for sometime. The tomatoes should be well cooked.
Mash the dal well with the back of the ladle. Add it to the boiled broth along with some salt.
Make seasoning. In a small pan, take ghee, add mustard seeds. As they splutter put asafoetida and turmeric powder, throw in the red chilly, a few curry leaves and pepper powder. Put the tempering into the rasam.
Once it boils, put lemon juice to it and remove from stove.
Lemon rasam is ready to serve.