In a pressure cooker, take dal with 1-3 glasses of water. Add turmeric powder, salt, sliced onions and diced tomatoes in it. Close the cooker and let the pressure release about 10 times on medium to slow heat. Let the cooker cool completely

Blend the cooked Dal till smooth. If you want to add veggies, add them at this point after blending. Add chilli powder, sambhar masala,l and coconut, but do not stir.

Heat oil in a tadka pan. Add dry chillies. Crackle mustard. Add asafoetida powder, curry leaves and the green chillies. Add it in the Dal over the spices. Do not stir yet. Close the cooker and allow it to whistle once or twice. Turn off the heat and let the cooker cool down.

Check the sambhar. If the consistency is thick, add a bit of hot water. Sambhar is supposed to be a bit thinner than dal. Add lemon juice and adjust seasoning as needed. If you added water and seasoning, shift it back to the stove and bring it to a boil. It's ready!