Heat oil in a saucepan. Once hot, add dry red chillies and let it roast until till chars and smokes a bit, about 10 minutes on medium low flame.
Throw in the chana dahl and sauté for 5 minutes until they turn crunchy and hard.
Add onions, tomatoes, garlic, asafoetida, tamarind, water and cook covered for 15 minutes. Add salt and let it cool.
Once cooled to room temperature, grind them to a smooth paste with avocado.
Make the tempering by heating oil in another saucepan. Once hot, add mustard seeds and let it pop. Throw in the dry red chilly, asafetida and curry leaves. Saute for a minute and add it to the avocado-tomato chutney. Check for seasoning. Serve with hot steamy idlis.