Soak tamarind in 2-3 cups of warm water and mash well. Extract the pulp in a bowl. Add chilli powder, sambar powder, turmeric powder, asafoetida and salt. Mix well. Heat oil in a deep bottom pan, add red chillies and mustard seeds and when it splutters, add urad dal and channa dal. Add onions and fry for a while. Add the vathal and fry until it turns brown. Add the tamarind pulp and mix well. Add sugar and let it boil on low flame until oil separates. Add curry leaves and the fried onion vadagams. For a thicker gravy, add rice flour mixed with little water and let it cook on medium heat for 2 more minutes. Serve hot with rice along with fried papad or potato fry (Click Here and here for recipe) to make it a beautiful combo. Add a tbsp of gingelly oil, if preferred! Enjoy!