Heat oil in a Kadai/ pan, add the whole dry Chilli & mustard seeds, once it crackles, add the onion & Chilli and saute. Don't brown the onion. Then add the tomatoes, curry leaves, little salt (you can adjust salt after you add the Milagai Podi), and Kashmiri LAL Mirch powder and saute.

Let the tomato cook, add the Milagai Podi, if needed add little water. When the tomato gets to cook and the oil separates, add enough hot water as needed, as the idlis will absorb the tomato gravy. Check salt. Add the Aamchur powder or tamarind whatever you have. Add the idlis and gently mix it. The idlis will absorb the gravy, if required add hot water but do not add excess or the idlis will become too soft and may break. Serve hot! Bon Appetite!