First, cook the moong dal with enough water to cover it by 2 inches, in the pressure cooker for 3-4 whistles or 15 mins until it's soft and mushy. Soaking the moong dal in advance helps.
Next, heat oil for the spice powder and reduce flame to low.
Add the ingredients to roast and keep stirring as the chillies start to glow and the dals turn golden brown. Take care not to burn them.
Cool, powder, and set aside.
Heat the 1 tbsp oil and add 1/4 tsp mustard seeds.
When they pop, add the chopped onions, perungaayam, and green chillies.
Fry until the onions turn soft (about 3 mins) and add the chopped brinjal and tomatoes. Fry again until the tomatoes turn soft (another 3-4 mins).
Next, add the tamarind water and turmeric powder and bring to boil. Cook until the tomatoes and brinjal turn completely soft (about 5 mins).
Add the cooked moong dal to this along with the powdered spices and bring to boil again. You can add about 2 tbsp of the spice powder, taste, and then add more if required.
Add salt and simmer until the right consistency is reached. It should be like a gravy that is pourable on Pongal.
Turn off flame and garnish with the chopped coriander leaves.