Add ½ cup rinsed toor dal and 2 cups water to the pressure cooker. Close the lid and pressure cook for 3-4 whistles. Let the pressure release naturally before opening the lid.
Once the dal has cooled, whisk it well. Add 2 cups of water and mix well.Note: You can strain the cooked dal if you prefer a smoother, more velvety texture, but it is completely optional. Heat ghee in a kadai. Add mustard seeds and cumin seeds, and let them splutter. Then add curry leaves and hing (asafoetida) and sauté for a few seconds until fragrant.
Add 1 cup of chopped tomatoes (about 2 medium tomatoes), and salt. Cook until the tomatoes turn soft and mushy.
Next, add tamarind water (or tamarind paste) and cook for another minute.
Add rasam pudi (powder), turmeric, and red chili powder (optional). Mix everything well.
Add the prepared dal water, jaggery (optional but recommended), and bring the saaru to a boil. Reduce the heat and simmer for 4-5 minutes.
Then add ¼ cup thick coconut milk (or ½ cup thin coconut milk), mix well, simmer for 1 minute on low flame, and turn off the heat. Garnish with 2 tablespoons coriander leaves and serve hot with steamed rice.