Heat a pan and dry roast the black peppercorns, dry red chilies and curry leaves util they are lightly roasted. keep this aside and let it cool down.
Wash the toor dal and cook this in 2 cups of water and salt until the dal is very soft so you can mash them with the spoon. You can cook the dal easily in a pressure cooker. Keep the cooked and mashed dal aside.
Grind the dry roasted spices along with one tomato in a blender with about 2 cups of water until the ingredients are very well blended. Keep this grind spice tomato mixture aside. This is the base for the rasam that we are going to make.
Heat 1 tablespoon of oil in a pan and transfer the cooked mashed dal to the pan.
Add salt and saute the dal for few minutes.
Chop the remaining two tomatoes in small pieces and add the chopped tomatoes to the dal in the pan.
Stir the contents in the pan and cook in low to medium heat for 1 to 2 minutes.
Add the remaining 1 cup of water, cover and cook for about 3 to 4 minutes. You may add a little more water if needed.
Add turmeric powder, cumin seeds powder and coriander powder, mix everything well and cook in medium heat for 2 to 3 minutes.
Add the grind tomato spice mixture and enough salt to the pan and stir everything well, saute for 1 to 2 minutes.
Cover the pan and cook the contents in low heat for about 3 to 4 minutes.
Once the mixture is cooked for 3 to 4 minutes, you may add more water if you prefer more gravy and cook for another 2 minutes more.
Add the chopped coriander leaves to the pan and stir well.
Now the tomato rasam is cooked well, let us prepare the seasoning for the rasam.
To make the seasoning, heat oil in a pan and add the mustard seeds.
When the mustard seeds splutter, add the curry leaves (make sure to there is no moisture content).
Pour this prepared seasoning over the prepared rasam, serve the rasam hot with rice for lunch or dinner.
You can also enjoy the rasam as a soup or stew even without pairing with rice, if you want to enjoy this as a soup, you may even skip the seasoning if you wish.