Soak toor dal for at least 1 hour. Drain well.Combine soaked dal along with the tomatoes, garlic, ginger, turmeric and water in a pressure cooker. Mix well and cook for 4~6 whistles or until dal is very tender. Alternately cook in an Instant Pot for 8~10 minutes on High Pressure.
Once the pressure comes down, open the lid and whisk the dal until smooth. Add tamarind pulp, green chilies, ground coriander, red chili powder and salt. If the mixture looks too thick, add more water. Mix well and cook on medium flame for 4~5 minutes.
Heat a small pan on medium heat. Add ghee and once it melts, add mustard & cumin seeds, dry red chilies and cook until the seeds start to splutter. Stir in hing and curry leaves and cook for another 30 seconds. Pour the tempering over the dal and mix well. Serve hot with rice or roti.
I recommend using fresh tomatoes for the best flavor in this khatti dal. But if you don't have any fresh ones on hand, then feel free to use canned diced tomatoes.
You can make this dal in Instant pot or a regular pressure cooker. Recipe card has instructions for both. But if you want to make this on the stove top, then combine the ingredients in a medium size saucepan. Bring the mixture to a boil, then cover and simmer until lentils are very tender. This might take about 30 minutes.
Leftover dal can be stored in an airtight container for up to 3 days. You can also freeze it for up to 2 months.
Sourness of tamarind varies from batch to batch. So adjust the amount added to the recipe accordingly.