Well I can bet there is no easy chutney to go with idli then this, I mean seriously no preparation at all, all you have to do is few ingredients and grind them that's about it. This is a my latest addition to my recipe repository.
1 Medium sized white onion cubed
1 Medium sized tomato cubed
Ginger about an inch thick
1 tbsp Cumin
2 tbsp lemon juice
salt according to taste
1. Grind all the above ingredients except lemon juice into fine paste.
2. Remove from grinder and add lemon juice and mix well.
Well that's about it, now tell me can it get any simpler than this. Enjoy with idli or dosa. You can also spread this chutney on one side of the dosa while making to give it a nice flavor.