First soak chana dal for 3 hrs.
In a blender, powder red chilli, cumin seeds, salt and asafoetida coarsely.
Drain water completely from soaked dal. Add to the blender and grind coarsely without water.
Transfer to a bowl. Add chopped onion, curry leaves and coriander leaves. Mix.
Make equal sized balls and flatten slightly in between your palms.
Heat oil in kadai, add the vadas 3-4 in a batch.
Cook in medium flame both sides until onion turn deep golden in colour.
Drain in paper towels and repeat to finish.
To make rasam, soak tamarind in hot water.
Extract tamarind juice with a cup of water.
Prepare rasam powder by blending chilli, pepper, cumin, coriander to a coarse mixture.
In a vessel, take tamarind extract, 3 cups water, turmeric, salt, tomato, asafoetida, rasam powder and few curry leaves.
Mix and bring to boil. Add prepared vadai immediately switch off the stove. Keep covered.
Temper by heating oil, splutter mustard, add red chilli, cumin and curry leaves.
Add to rasam and garnish with coriander leaves. Leave it closed for 15 mins minimum.