Take a non-stick pan and add oil in it. Heat it over medium flame. Make sure that the oil does not get too hot. Then, put some mustard seeds in the pan and wait for them to splutter.
Once seeds start spluttering, add onions and saute till they soften or turn slightly pinkish in hue. Now add tomato, cabbage, capsicum and saute for a minute over moderate flame. Sprinkle some salt and ketchup.

Take the leftover idlis, cut them in 4 pieces and add into the mixture. Mix it well and garnish with green coriander. If you have sudden guests coming over and you cannot prepare an elaborate meal, this is your go-to recipe as it takes very less time. Serve hot with a hot cup of tea.