Heat oil in a cooking vessel, add the mustard seeds and let them splutter. Add the curry leaves and saute till a nice aroma emanates the kitchen.
Add the green chilies and ginger and saute for few secs.
Add the chopped onions and saute on medium heat for 3-4 mts. Add turmeric powder, red chili powder, coriander powder, saunf powder and garam masala powder. Mix well.
Add the chopped tomatoes and saute for 5 mts till oil separates. Add the cubed idlis and mix. Saute for 2 mts. Add lemon juice and mix. Turn off heat.
Garnish with chopped coriander leaves. Serve warm.