In a 3 litre pressure cooker, cook the lentils with the tomatoes, garlic, ginger, turmeric powder, 2.5 cups water for 7-8 whistles or more till the lentils are tender and mushy.
When the pressure settles down naturally in the cooker, then only open the lid.
Mash the dal with a madani, wooden spoon or hand blender till fine and smooth.
Meanwhile when the dal is cooking, soak tamarind in hot water for 20 minutes.
Squeeze the pulp from the tamarind directly into its bowl of water.
Strain the tamarind pulp and add to the dal.
Also add the green chilies, turmeric, red chili powder and coriander powder. Stir in ¾ or 1 cup water along with salt.
Keep the dal on a low heat and simmer for 8 to 10 minutes or till the raw aroma of the tamarind goes away, all the flavors have blended well and the dal has reached a medium consistency.
Cover and keep the cooked dal aside.
Making tempering
Heat ghee or oil in a small frying pan. Keep heat to a low. Crackle the cumin seeds first.
Then add the garlic and fry for 4 to 5 seconds on low heat. Add the dried red chilies, curry leaves and asafoetida.
On low heat fry till the red chilies change their color and the curry leaves become crisp.
Pour this entire tempering mixture over the khatti dal and stir to combine.
Cover and keep aside the dal for 5 to 6 minutes for the flavors to mingle.
Serve Hyderabadi Khatti Dal with steamed basmati rice, jeera rice, saffron rice or roti.
Notes
For cooking the lentils in a pot
Rinse and soak the lentils for at least an hour.
Boil 3 to 4 cups water with a pinch of turmeric. Add the drained lentils.
Whilst cooking remove the scum with a spoon. Cook the lentils till the lentils have become soft and mushy. Add more hot water if required while cooking.
Once the lentils are cooked, you can add the tomatoes, etc. and simmer till the tomatoes are cooked and then proceed with the recipe.