Drain urad dal and grind to a thick batter. Add coconut, green chillies, ginger and salt and whisk the batter well.
Heat sufficient oil in a kadai. Dampen your hands, shape the batter into small balls with finger tips and drop into the oil. Deep fry till golden. Drain vadas on absorbent paper.
For rasam cook the toovar dal with one cup of water in a deep pan till it gets mashed. Add tomatoes and one cup of water and salt and let it cook till the tomatoes soften.
Heat oil in a pan. Add asafoetida, mustard seeds, broken red chilli and curry leaves. Strain the tomatoes and dal and transfer into a mixer jar and grind to a smooth puree. Add the remaining stock to the tempering and mix.
Add the puree and mix. Add rasam powder, tamarind pulp, jaggery and salt and mix well. Add coriander leaves and mix. Simmer for two to three minutes.
To serve place the vadas in serving bowls, pour the hot rasam over and serve immediately.