Chop the idli and keep them aside in a bowl or plate. Make bite sized pieces and do not chop finely.
Finely chop 1 medium sized onion and 1 medium to large sized tomato. Keep them aside.
Also chop 1 green chillii. Keep aside.
Also crush ½ inch ginger + 3 to 4 garlic cloves to a paste in mortar-pestle. Keep aside.
Making masala idli
Heat oil in a pan. Add cumin seeds and let them crackle on low to medium-low heat.
Then add ginger-garlic paste. Saute for a few seconds or till the raw aroma of ginger-garlic goes away.
Then add finely chopped onions.
Stir well and saute on low to medium-low heat till the onions turn translucent or a light brown. Stir often while sautéing onions.
Now add finely chopped tomatoes.
Stir and mix well. Then add chopped green chilies. Saute till the tomatoes soften on low to medium-low heat.
Then add turmeric powder, red chili powder and pav bhaji masala. Also add salt as per taste.
Stir and mix the spices very well with the onion-tomato mixture.
Add water. Use idlis which have been refrigerated. If using fresh idlis then don't add any water. You can even skip adding water if you want.
Mix well and simmer the gravy on a low to medium heat.
Simmer till the gravy thickens. This takes about 2 to 3 minutes on a low heat. Make sure that the gravy thickens. If the gravy is runny or watery the idli will soften and crumble.
Then add the chopped idli.
Mix the idlis very well. The idli will absorb a bit of the thick gravy and become moist and soft.
Switch off the heat and lastly add chopped coriander leaves. Mix well.
Serve masala idli hot or warm. You can sprinkle lemon juice if you want. I recommend to have this dish as soon as it is made for best taste and flavor.