Cook dal as per your convenience adding 1 cup water. I normally cook dal in the steamer.
Boil tomatoes (if using), asafoetida and salt in a vessel. If you are not using tomatoes, boil all in a cup of plain water.
Roast urad dal, red chilli, cumin seeds and pepper in very little oil till dal becomes brown and grind to a powder.
Take only the water from the top of the cooked dal. Dal can be used for any other purpose. Add powder to the dal water and add this to the boiling rasam. If you feel its very spicy, add part of the powder. The rest can be stored.
Add more water if required.
Bring it to just one boil and switch off the flame.
Make the tempering and pour it on rasam.
Garnish with coriander leaves and serve hot.
Notes
This powder can be made in large quantity and kept for use any time. This way, this rasam can be made in jiffy.
You can use tomato or just squeeze a lime if you like it sour.
The spiciness of the powder depends on the chillies you use. So use your discretion when adding it to rasam.