Rinse and soak the toor dal or arhar dal for 10 minutes. Pressure cook the dal with sufficient water. Mash it well and keep aside.
Heat a pan, add oil, mustard seeds. Once mustard crackles, add cumin and whole dry red chili, hing (asafoetida).
To this, add chopped tomatoes, curry leaves and turmeric powder.
With the back of your spoon or ladle, gently mash the tomatoes.
Once tomatoes appear mushy and cooked, add 4 cups of water.
Allow it to simmer, then add rasam powder, red chili powder and salt to taste.
Once it reaches a boil, add the cooked, mashed lentils (toor dal). Mix well.
Check seasoning (add extra spices, salt, if required). Let rasam simmer on low heat. Once it froths up, switch off the flame, add lime juice, jaggery, finely chopped coriander.